DISCLAIMER: I work for Nokia, Forum Nokia.
DISCLAIMER 2: What I am going to say here is exclusively my own thinking and analysis and based only on what is publicly known.
I have no insight on why and how the decisions were made, no insight of any behind-doors-agreements, gossip or promises.
What I am writing here is simply the result of my thinking based on what I have read and heard in the public announcements.

The options

Symbian

Symbian is an old operating system that was developed for pocket devices, what we once called PIM‘s or PDA‘s. It was designed for devices that had a keyboard or a keypad, that were low on CPU and battery.
For these reasons Symbian is VERY efficient on battery and CPU. Most of the UI was crafted to work well with a keyboard and a joystick. You might not know it, but in your Nokia N8, designed to be entirely touch and with no hardware keyboard, you still have a lot of keyboard shortcuts that you will likely never use. The Nokia E7 that is finally reaching the shelves now has a lot of shortcuts that you can use with your qwerty keyboard.
The N8, with a 680Mhz CPU, performs almost as well as most Android devices that use a 1Ghz or better CPU, because it’s highly optimised, but you will never know it, because the UI is too clunky (more on this later).

Unfortunately what users expect at the high-end, is not what Symbian can offer today. Many of us are used to recharging their mobile every evening, something that most Nokia customers don’t do. Modern high-end mobile devices are expected to be snappy, to have a great browser with device APIs and graphics hardware acceleration, support applications developed with a relatively high-level programming language that lets you create a decent application or game in a week or so.
Symbian was not designed for this.
Symbian^3 has a much better UI than before, mostly designed to support touch, but still with a lot of legacy menus and interfaces. The programming language is C++, with a lot of quirks and strange libraries. This is the result of 10+ years of evolution in a direction that is not what was planned back then. If you know Symbian and C++ very well, your application will have a raw performance that most other OSs can dream of, but unfortunately it will take you 3 months to develop, and not 3 weeks.
This is true for developers out there, but it is also true for Nokia. Despite how hard Nokia has tried to blow new life in Symbian, it failed because the development environment is just too complex.

Unfortunately for Symbian, its highly optimised environment has determined its demise.

MeeGo

MeeGo was developed from the positive experience of Maemo. Maemo was an experiment, but it showed a lot of strong points and a surprisingly vibrant community. MeeGo was built on the best intentions to bring a great new OS built for mobile devices of all types (netbooks, tablets, in-car entertainment, smart phones).
Unfortunately it has not delivered in the time frame that was needed to Nokia not to lose too much market share. iOS and Android devices are selling well, too well to keep waiting. Apple has iPods, iPhones, iPads and appleTVs. Android has loads of mobile phones, is coming to the tablet world and has the Google TV.
Nokia announced in 2010 that it would deliver at least one MeeGo device. Then very late in 2010 announced that it would arrive in 2011.
So far, we haven’t seen anything.

MeeGo is still a great promise, but unfortunately Nokia needs to sell devices today in order to develop a better OS, apps and platform for tomorrow.

WebOS

WebOS, is in my opinion a great OS with a very interesting approach with Web technologies as its development platform. It’s a great idea, that hasn’t really delivered for Palm. HP seems to be betting really big on it with the recent announcement of some very interesting devices in all ranges. That is fantastic for WebOS and HP might be able to work around the operators selling directly to the thousands of medium and large enterprises that rely on them for their servers, networks, printers and computers. It will be a great synergy I’m sure.

What does that offer to Nokia? Well, Nokia might have embraced it, but was HP interested? Probably too busy developing its own devices and integration with all the other services. HP certainly has a grand plan and Nokia might have been a distraction more than a valuable partner.

RIM with QNX

Similarly to HP, RIM has a foot in the enterprise business and something also in the consumer business. It has completed some very interesting acquisitions such as Torch mobile (which resulted in a much better browser in their blackberry OS 6) and TAT. They announced the Playbook tablet (a little too early IMHO) and are busy trying to get QNX to work on their devices starting with the Playbook tablet and later on mobile devices.

Similarly to HP, RIM is probably too busy completing its plans, to be bothered with Nokia. Let’s not forget that Nokia can be a great partner, but will also come with some strong requirements and big personality. At the end of the day Nokia has the infrastructure and experience to sell millions of devices per day, it cannot settle with a partner that will not have time for it.

Microsoft and Windows Phone 7

Of all the mobile OS’s installed on devices that are on the market today Windows Phone 7 is the youngest and likely the one that still has to prove the most. It has a nice new UI, very consumer oriented and it has the Microsoft hat, so you can expect good integration with their enterprise products such as Office and Exchange.
Yet, the browser is not great and while the development environment is the very well known .NET, it does not have a number of developers dedicated to this platform that can make their store very interesting.
Microsoft has been able to convince LG and Samsung to produce mobile phones and that’s great, but the sales so far haven’t been amazing.

Android

Assuming that Apple was not keen on letting Nokia use iOS, Android would have been the most obvious choice. Android is the most mature “modern” mobile OS after iOS, has a good UI, the second best marketplace and a vibrant, growing community of developers. I imagine that the conversation between Google and Nokia went like this:
Andy R.: “Hello Stephen, nice to see you. So you are interested in Android? Well, we think it’s great, we have a great OS, we have a fantastic UI and we have some of the best UI and UX expert in the world working to make it even better. Our developers love our open source attitude and love the Google cloud services. We have by far the best search engine, the biggest ad network and the best maps. You are welcome to take the source and run with it. What can you do for us?”
Stephen E.: “Wait a second, WE have the best maps, ours are vector based and much faster to download, plus we can preload them”.
Andy R.: “Stephen, we have vector maps as well, already deployed on Android 2.3 and even better in 3.0″.
Stephen E. stands up and walks out with a little teardrop.

What Nokia has on the table

So what could Nokia offer, as a partner, to these players that would make the cooperation more interesting for both? Nokia is by far the number one player in the low-end market and in most if not all developing countries. In India, China and South Africa Nokia is basically a monopolist. This is very cool and very good for Nokia’s cashflow, but not of great value for these high-end operating systems. Android is going down the chain of value, covering more and more segments, but it’s still not in the 50-70 USD price range where Nokia rules. All the others have requirements not even close to that price point.

Nokia has Symbian and a large portion of users that today have a Symbian device and will likely buy another smart phone in the next months. Providing a smooth transition is a key selling point (anyone wants to manually enter all their 1200 contacts in the next phone?).

With both the low-end and the high-end device segments come a lot of developers. It is true that Nokia is not the number one platform to develop today but being the number one vendor for more than 10 years means having contact with thousands of mobile experts. That is worth something.
Forum Nokia is of course the heart of the Nokia developer community and that means millions of pageviews per month.

Nokia has acquired NAVTEQ and maps have been an integral part of its strategy for the last couple of years. This is actually a GREAT asset as there are basically only two providers of raw cartography, Nokia and TomTom. With those come points of interest (shops, hotels, petrol stations, etc) and years of experience in navigation, of course.

Last but not least there is the Qt development framework. Android and RIM use Java, Microsoft uses .NET and WebOS is, well, Web. Apparently not very interesting, but don’t forget that Qt developers are normally highly skilled developers breathing Linux and open source software, writing cool hacks and advanced C and C++ in their sleep. That has a GREAT value for anyone that dreams of creating a healthy ecosystem with cool applications that will sell well and will attract customers to their platform.
Along with Qt comes QtWebKit, a port of the great WebKit engine to wrap in native applications. Anyone not taking that into account would be underestimating the power of Web technologies and the desire of Web developers to monetise their content.

Nokia also has a number of services, more or less successful such as Music, Share, contacts and social. I will not go into the details of each, but of course consolidating the users in any existing online service would be great for anyone. Think about Blackberry messenger on 200 million Nokia devices, or Microsoft Live messenger, or finally having someone to talk to on Google Talk. Although none of these is a critical element, they all add to the overall value. A value that Nokia does not want to throw away.

Conclusions

The Microsoft deal

So the answer to Nokia’s hunt for a partner is not necessarily who is the best operating system or platform, but rather which one is the best partner. Which one of those companies will get the greatest benefit from Nokia’s assets and which one will bring the greatest value to Nokia?
Nokia has survived more than 150 years going through multiple mutations, jumping from a market to another, building tires, televisions, PC monitors and boots and eventually mobile phones. While the main focus for the past 15-20 years have been mobile phones, Nokia has diversified its business, it owns NAVTEQ, has a foot in the music distribution business, a foot in advertisement and can provide a great platform to expand pretty much every possible business.

In conclusion Microsoft is the best partner for Nokia because not only it provides many pieces of the puzzle, but it also needs many other pieces.
The partnership with Microsoft will bring great integration in the Maps, IM, advertisement, search, location and more. Both Nokia AND Microsoft will give something to the relationship and both will get something out that they did not have before.
It is going to be tough because these are both very large enterprises that have been working on their own for many years and working together will be hard, but they probably both want to make this work.

The backup plan

Let’s not forget that although Nokia is now going to spend a lot of R&D resources to get as many WP7 devices on the market as soon as possible, it will also keep alive MeeGo and Symbian. If the partnership does not work, both parties can walk away and still might retain what has worked well.
This is what today looks like the most likely scenario for a win-win for both companies (yes, I am being cautious).