(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.
3. How to Break Into the Arcatraz. And here's the questline for the Arcatraz key. It starts with a quest from Spymaster Thalodien in Area 52 called "Assisting the Consortium," and will take you all over Netherstorm, and eventually into both of the other Tempest Keep instances-- Botanica and the Mechanar. It's longer then the Shattered Halls key quests, but I like it just because I'm such a big fan of the Ethereals.
4. Ashtongue Deathsworn. Of these five questlines, this is the one I enjoyed the most, probably because it takes you pretty close (at least until the 2.1 patch) to someone you were "not prepared" for when the expansion came out. Lots of great lore and story here, too: the Ashtongue Deathsworn are a group of Broken in Shadowmoon Valley who are working for Illidan-- or are they? To begin this questline, you have to visit the Aldor or the Scryer outposts in Shadowmoon, and get a quest called the Tablets of Baa'ri.
5. Manaforge Destruction Duty. This long questline earns you a lot of Aldor or Scryer reputation (depending on which one you're loyal to, obviously), and takes you around the Manaforges in Netherstorm, huge mana power plants that are run by one Kael'thas Sunstrider. In this questline, you'll disrupt operations at all the main manaforges, and eventually discover a very sinister secret about what Sunstrider has been doing in Outland. To begin the line, visit either an Aldor or a Scryer representative in Netherstorm's Area 52.
By the time you finish all of those, you'll have picked up some great quest rewards, earned a lot of gold, and you'll be ready to raid the endgame. Also, you'll probably be quested out-- until the next patch is released, anyway.