(And yes, I do realize that most hardcore players will have already found and completed these chains-- if you know of more fun, rewarding, or cool quests to do post-70, feel free to add them in the comments below.)
1. The Cipher of Damnation. This is a loooong, long questline that will get you ready to do The Eye, the 25-man raid in Tempest Keep. It starts off with some solo quests in Shadowmoon, and then takes you through all the Heroic instances in TK. Along the way, you'll use boars to dig for tubers, and puke your guts out going through rotten Arrakoa eggs. Lots of fun. It starts in Shadowmoon Valley's Wildhammer Stronghold if you're Alliance, or Shadowmoon Village if you're Horde.
2. Entry Into the Citadel. This questline and the next one are key quests-- they'll finish with key rewards that will let you into endgame dungeons. This questline is the making of the Shattered Halls key, necessary to enter the toughest 5 man in Hellfire Citadel (not every group member needs these keys to enter these instances, just one-- unlike the raid instances). So lots of guildies will love you if you finish these quests, and for this questline, you get to kill a Fel Reaver in the process. The Shattered Halls key questline starts from a drop from Smith Gorlunk, on the Northern Terraces outside of the Black Temple in Shadowmoon Valley.