It might sound very insignificant, but... (Score 2, Insightful)

One of my favorite features of an IM client (which is present in both Google Talk and Adium) is the automatic grouping of multiple IMs from the same person. For example, if I type 5 lines before you type another, it only shows my name once with all 5 messages. I think it makes reading conversations much more pleasant :)

Here's a screenshot of how Adium does it:
http://www.adiumx.com/screenshots.php?show=overvie w.jpg