Ti is wonderful! Light weight, springy, no scratches, no corrosion...what more could you want? I don't understand how anyone could not want a ti frame!

Stuff I haven't tried:

Disc brakes...and I'd really like to give them a shot. I've read the good and bad, and want to see for myself.
Electronic shifting...I'd love to give it a whirl, though the current price tags make that unlikely.
Carbon Fiber...I haven't really ridden CF much; a few rides here and there. I like CF forks and would like to try a frame.
Wheels under 32h...no thanks, I like my wheels tough, and I like some ballast under me when going fast.
Tri bars...look awful, no interest.
Recumbent...I'd love to try one, but not on city streets. I like being seen.
Full suspension MTBs...I'm far too timid to need one.

The only 650b I've ridden is my tandem, and I'm not sure if that's representative. I love the wheels on it though; very smooth, roll well over grass and cobbles, they feel like they spin up quickly.