One thing I am continually annoyed with when working with the Visual Studio test framework is that, when a test fails, I have to look at the error to figure out what line of code in my test method caused the problem, then separately find that class, browse to the line, and figure out what's going on. This isn't how we do our development, so why is it how we do our unit test development? I know this was a v1 release -- although I hate that excuse -- but I hope we'll see some significant improvements in Orcas.
Edit: Looks like this was reported on VS 2005 beta 2 
and ultimately closed with a claim that it was added to the RTM release. Obviously, it wasn't, so I added another suggestion . We'll see what they say.
Edit: According to the folks at Microsoft Connect, this will be in VS08. I look forward to it!