There's been a lot of developer frustration working with SharePoint but we all hope Microsoft has been listening. With that in mind...
What updated and new features for developers excite you about working with WSS 4.0 and SharePoint Server 2010?
One answer per feature please so the community can vote on each one.
They say they've streamlined the performance so I'm anxious to see the results of that. I don't know a user or developer who enjoys using SharePoint. Prior to 2010 it's the most bloated website I've ever used.
Official LINQ support. Seriously, CAML is stupid, and replacing it with LINQ in most cases will be a blessing.
The new Client Object Model (OM), which will make it a lot easier writing SharePoint apps running on remote machines. It looks a lot easier than working with the current set of inconsistent and cumbersome Web services.
My personal 'want' is the developer dashboard!
Ability to add Custom List Forms for the List using the InfoPath. Also you can Specify the validation logic for the Fields.
Visual Web Parts: Design-time experience for web parts. No need to dynamically create controls or use kludgy user control workarounds. This should ease the pain ASP.NET developers find getting up to speed with SharePoint.