Definitely my favorite part of BC so far has got to be the instances. We haven't seen new 5-man content in such a long time, and it's so much fun to be facing new pulls, new lore, and new bosses, most of them with brand new models, and even audio! Although, while I do like Magtheridon's voice, it seems like yes, those "puny magics" are actually holding him for a long, long time-- we get it.
Most of the new strats aren't too hard to get down with trial and error, but there are a few quirks to figure out. And for that reason, my guild's raidleader, Belacroix of Gothic Bunnies on Thundehorn-H, has punched together a few quickguides of the first few instances-- two wings of Hellfire Citadel, and The Slave Pens and the Underbog in Coifang Reservoir. They're not complete walkthroughs by any means, but if you want a little insight on how to take down certain bosses or what you'll find in there, Bela's guides are a good read.
WoW blogger Relmstein has also been running instances in Outland, and he's got even more complete walkthroughs for Hellfire Ramparts and the Blood Furnace (so far, the Furnace is my favorite). And I have to make a mention of WoW Wiki as well (and everyone that works on it)-- they've got strategy up for almost every boss seen in the instances so far, so if you've got a group stuck on something, they're the place to go.