The difficulty with the N900 was that they introduced one phone. As an N900 user (still) I know that the N900 is not for everyone, and was targeted more at the tech-savvy user than your bog standard just-want-a-phone-that-works user. Not everyone wants a built in keyboard or a phone that large. In fact, one of the reasons I haven't changed phone is that it is so difficult to get a decent phone with a slide-out keyboard, similar the the N900. Suggestions welcomed...
Nokia needed to produce several phones around a similar theme aimed at different users, or do what Apple did and produce one phone to a very high standard aimed at the average user, not at a niche market (80/20 rule). Doing what they did was narrow minded and poor business sense.
Nokia's support for developers was, frankly, not an enjoyable experience. This is where being part of the Windows universe will be an advantage to Nokia as that will be Microsoft's responsibility, not their own.