These familiar faces are spread all throughout Northrend, and we've collected a lot of them in a gallery for you to look through. A few of the returning characters are missing from the gallery for various reasons (like turning in an unfinished quest despawning all NPCs in the area) but we'll get those when we can, too. There's plenty of spoilers in this gallery, though not many of them are massive plot points. Still, if you want everything to remain a secret, I'd be careful.
Oh, before all of you spam the comments about it, no I did not forget 'the Scourge.' I thought that was a little obvious. Also, there are two massive zones and numerous dungeons not yet implemented. There are probably way, way more to come. We haven't seen Sylvanas yet.