Just to summarize a few from the Titan forum over at XDA:
Callers can't hear you half the time
After loaded a certain amount (or certain) of apps, the phone becomes laggy playing video/games
WiFi connection integrity issues
They've already got a hold of some official (but unreleased) service center rom and they're talking about flashing it in and if it fixed certain bugs. That just gave me flashbacks to the current WinMo phone that I use, and used to flash monthly.
I want to switch to WP7 for smooth, bug-free operation. Not an experience like WinMo or Android.
I'm not totally dissuaded but I am keeping on eye on issues at the same time.
Have to keep in mind the Titan has been out longer than the Focus S (in my mind, the only real competitor) and to an international user base, at that. So I need to take that into consideration.
That's why I am hopeful that AT&T will at least make sure the phone is bug-free for their user base. I have no idea if this is something they care about, or if they only do network validation testing.