Tabbed user interfaces
I've been using Firefox for web browsing since it's first release, so I'm quite used to tabbed-browsing.
The new version of Firefox (and IE7 too) has made a small change to how tabbed browsing works - the 'close tab' button is now part of each tab.
What I've realised is that there's a difference now with how Visual Studio works - it's tabbed editing environment is like the old Firefox method - there's just one 'close' button over on the right-hand side.
I'm not sure which way I prefer - I can see advantages to both, but I just wish that I could make all the apps do it the same way and not be inconsistent now.
I haven't tried any of the CTPs of the next version of Visual Studio yet, but I would guess that they'll change Visual Studio to mirror tabbing in IE7 (and Firefox 2).