I've ordered Runebound at my local game store and anxiously await it's arrival and subsequent perusing of the rules to begin play. I've been obsessing about expanding on it already after a few playthroughs. I love the idea of the expansions on keeping the game fresh. ( I loved HeroQuest and Gammarauders back in the day). I understand the general expansions, but I'm unsure of the card expansions, notably the "class" decks. If I could get some enlightenment on that, great! I'm thinking already of expanding into other games as time goes on...
The class decks are designed to increase PvP play in the game (which is pretty minimal otherwise.) I don't own any of them myself, but as I understand it, each player needs one Class deck in order to use them at all. Players are also allowed to "invent their own class" by mixing and matching cards from different class decks, assuming you have enough such decks to go around.
A lot of the cards therein allow you to poke other players from across the board. Throwing your own challenges in front of them, applying penalties to their next roll and so on.
The other card expansions add more challenge cards, more items/allies, etc. The Adventure Variant decks provide a whole new victory condition and often new rules during play. Those are easily my favourite type, personally.
Basically all of the card expansions are OOP these days, although FFG has recently released an Essentials box that has 6 of the more popular card expansions in one. Hopefully more Essentials packages will follow, but if you see a single card pack you want at your local store, you should probably jump on it before it disappears.