Only a day ago, I was reading a quite detailed article about the new features and improvements in WP 2.9 — this article I can recommend to you, too.
I liked two things more than others:
— You can already restore items from the “Trash” — items that you have deleted by accident or on purpose! Everything deleted will be kept in the Trash for 30 days (and you can change the time period for which the items will be saved there).
— Mass upgrade of plugins with one click only!
Of course, there are a lot of fixed bugs and some other improvements (full list).
Although I already feel enthusiastic about 2.9, I might wait for 2.9.1 to be released… sort of a Service Pack 1 for 2.9.x ;-)