I have a confession to make; I love fishing. Most people seem to hate it, and it's certainly not one of the more popular professions to level, but I could pass a perfectly entertaining afternoon ingame doing nothing more than chatting on vent and emptying pools of delicious and lucrative fish. Blizzard made a lot of effort towards improving the fishing experience in Burning Crusade, and tons of people wound up leveling it for a shot at Mr. Pinchy, the Find Fish skill, or the high-end raiding food buffs like Spicy Crawdad and Blackened Sporefish.
You'll find an array of new fish and pools in Wrath, but for my money the two great joys are fishing in the Dalaran sewers and the Dalaran wishing fountain. The fountain is actually the more interesting of the two for a reason we'll cover in a bit, but you'll also need the sewers if you're interested in a new noncombat pet and the Accomplished Angler achievement (the one rewarding the "Salty" title, which I'm sure no one in the game ever intends to abuse).
Spoiler warning: if you don't want to know the disgusting but highly interesting things you'll coax out of the Dalaran sewer system, just keep scrollin'.