In general or just for mobile users? I could write a book about how to improve this site, but if you want feedback for just mobile users:
The site should detect when you're using a mobile device and have a better quick reply box. Quick reply on the phone can be frustrating to work with. It's even worse with PMs, as a box appears over top and sometimes and it's difficult to hit "reply" without it trying to register a click behind the reply box. It would be nice if they had some quick links to people's most clicked spots like the postlist, your buddies, PMs, etc. Fat buttons for phones instead of magnifying the phone until you can hit one of the miniature buttons.
A government big enough to give you everything you want, is strong enough to take everything you have. -MLK