Add Reference dialog in Visual Studio 2005
In VS2005, the dialog changes to only a single, multi-select list (a single list per tab, that is). This is bad. Multi-select lists have poor usability - especially for very long lists (and the default list in 2005 is about 9 pages, which qualifies as long in my book). Selecting multiple items from disjoint sections of the list is difficult, and its easy to accidentally unselected something that you selected three pages back with an errant mouse click. Adding both COM references and .NET references multiplies the problem - now you multiple multi-select lists. I don't know why MS decided to make this change - in my mind, the 2003 version was much more usable.
On the other hand, the references dialog now includes a "recent" tab, listing recently used references. This is a definite usability improvement.