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.
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.
I would have to say clean XHtml output. That has been bugging me for the 3 years i've worked with sharepoint.
Development support on Win 7 / WS08R2
You no longer have to do your development on Windows Server. You can use Win 7, Vista, or WS08R2.
Dev tools support
SharePoint is now a first class citizen in Visual Studio. You can throw away WSP Builder and Sharepoint Manager, both great tools for SP2007
Business Connectivity Services (BCS) - In MOSS it was known as BDC - Ability not only to read the data from External data Store, but to write back. As show in the demo video there will be a List of type for External Data, that will let you to connect External data store and automatically the Add/Edit form gets rendered.
My personal 'want' is the developer dashboard!
XSLT based Rendering for the List Views
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.
Correlation ID's for errors
Small change, but lets you search the logs folder for a correlation ID A correlation ID for errors. You can use this ID to view the error in the Logs folder.
Powershell support
Anything you could do with STSADM can now be done by using the fully power of powershell. There will be feature parity between the two.
The sandbox.. being able to deploy custom code to SharePoint Online and other hosted solutions will skyrocket SharePoint usage
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.
Ability to add Custom List Forms for the List using the InfoPath. Also you can Specify the validation logic for the Fields.
More control over Sync Event Handlers:
Currently in 07 We have an option to cancel the event set the ErrorMessage that will be displayed in the Plain Boring Page of SharePoint. It has been improved in MSS 10 ,now that we can set the redirect Page to which we want to redirect the user to.
来源：https://stackoverflow.com/questions/1491513/what-developer-features-excite-you-the-most-for-wss-4-0-sharepoint-2010