Working with the ASP.NET AJAX framework has been fun. I think the basics are well covered for the most part, and beyond that, your imagination can likely come up with some really interesting UI.
What's frustrating, however, is the way it has become a moving target. There's the original release, 1.0, as an extension to ASP.NET v2. Then there's v3.5, which comes in .NET v3.5, which includes ASP.NET v2. In the background, we had the "futures" package, and now the forthcoming v3.5 SP1, which has better a version of the history control, so don't use futures. Oh, and the roadmap for v4 was more or less recently released. It's madness. (Oh, and the documentation on MSDN, linked to from the old v1.0 docs, is poorly organized compared to the old stuff.)
So if you're a developer trying to get up to speed, where do you start? What's really important? For me it's easy, because I do independent stuff that interests me, and I happen to be between regular day jobs. I've been pretty focused on v3.5 and doing useful things with it, like a Facebook-esque auto-complete list (puts a "token" representing a data item in the box that you chose via auto-complete).
In the corporate world, adoption of anything new is slow. This makes me wonder... who are the people using the latest bleeding edge stuff, and why? Sure, they're the bloggers and authors and such, and the new open Microsoft is getting their feedback, but are they the right people? There are a whole lot of smart people involved, but I wonder if the chaos of versions and service packs and futures and community driven stuff are well connected. (The ControlToolKit's ReorderList control throws script errors in IE of all things for me, which doesn't instill confidence.)
Sometimes it's hard to just focus on what you have in front of you. I guess with all of the excitement over various pieces of the framework, the latest being the MVC piece, I worry that there is so much going on that there is risk for ASP.NET to become a convoluted mess, and that would suck.