One Great Quest Chain

Everyone leveling Alliance in Wrath should definitely ensure they complete a particular quest chain. I've linked the whole quest listing start-to-finish here in case you need it, and this is the very first quest if you're brand new to the zone and don't want to read the later quest-names.

I'll do my very best to pitch the quest without spoiling the content. It entails...

1) An extremely long (but cohesive) chain in both Dragonblight and Azeroth, there are a total of 20 quests to do.
2) It's also an example of how dramatically Blizzard has improved the quest system, it is an extremely accessible chain that is fun, varied, and doesn't involve tremendous travel.
3) You get to interact with some notable NPCs through the chain, including Bolvar Fordragon, King Varian Wrynn, Jaina Proudmoore, Alexstraza of the Red Dragonflight, and even Thrall and Sylvanas as well.
4) You get a lot of experience with the new vehicle system, you get to pilot gryphons, drive a steam tank, and assault the scourge on the back of a dragon.
5) It includes the first ever "in-game" cutscene. While it is not an FMV it is much more sophisticated than the typical "stand there and read two characters spamming /yell at each other" in the past.

While the famous (and now removed) "Great Masquerade" Onyxia quest will long live in people's hearts, this chain (which I'll refer to as The Wrath Gate chain until/unless the internet settles on a superior one) is an amazing (both in gameplay, lore, and technique) creation in WotLK.