The old UI is what kept me from using Wave in the first place. I use Opera, and would need some pretty dramatic incentives to switch (cash works - I'm willing to use an e-mail client via Firefox at work). Of course, the current design could be improved upon, but not by simply regressing to the old version.

Incidentally, browser incompatibility with Opera also stopped me from using Quora.

Some of the UI features didn't get released when wave was open sourced, Google not wanting exact duplicates of themselves running around.