I realize we hardly know too much about the set, but now's the time to start speculating, talking, etc.
I personally refer to the spoiler list from MTG Salvation, which has a whole two cards listed right now. (
http://mtgsalvation.com/return-to-ravnica-spoiler.html ) I'm already semi-excited for it since I loved the Ravnica block already, and going back just seems more awesome.
I am sad that I don't get Boros (red/white) until the exapansion Gatecrash, but Selesnya (white/green) will do only because I have a white/green human-based deck which is already ready for rotation. (All I need are the Neahearth Pilgrims to replace the Suture Priests. Which are easy as hell to get.)
Sure, when RTR hits, I lose all of Scars block (Scars of Mirrodin, Mirrodin Besieged, and New Phyrexia), but I think RTR will make up for it. Bonus points if it makes my human deck more counter-based silly than it is. ;)