You'll lose Classic environment support, so if you're still hanging on to some legacy apps, you might want to reconsider.
There are emulators out there so you can load os 9 like you would in classic in tiger, so all is not technically lost.
You lose the obvious things too, like the white menu bar, and the rounded corners at the top.
But you get a lot of new features too, which is nice, if you'll actually use them.