You probably already know about the Fires of Outland set, coming on August 22nd (that's the one that has the Spectral Tiger Mount in it), but we were also told about the next set release, which will be called March of the Legion, and will drop in November. There will be new loot cards for that set (we've haven't been told what they are yet, but when we hear, so will you), as well as the introduction of "sub-factions" (I'm assuming this is factions like Scryers or the Mag'har), demonic allies, and new powers for Heroes. Also, each booster pack in this set will have 19 cards instead of the current 15, and 19 cards will be the standard from now on. More cards for the buck!
The other new set we heard about is a special holiday set, also in November-- Upper Deck is going to release a Feast of Winter's Veil collector's set, and it'll have stuff from the ingame holiday to use in the TCG, as well as 10 exclusive game cards (not sure what that means), and Pet mini-cards. As far as we were told, the collector's set won't have any loot cards in it, but if you're into the TCG (or know someone who is, as they're marketing it as a gift set), maybe it'll pique your interest.
At any rate, the thing we're most excited about, of course, is the new loot cards coming with March of the Legion, which hopefully we'll hear deets on soon. I still haven't quite been sold on actually buying these things-- the card game is fun, but all of my WoW spending money is going to playing the MMO at the moment. Still, if you're into it, a new set is always exciting to hear about.