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 theDalaran 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'.
El's Extreme Anglin' recently posted a fun little video previewing Fishing in Wrath of the Lich King. Specifically, in Dalaran. I have no idea what draws people (including myself) to such a boring profession, but this video shows a bit of what Blizzard has done to make it more entertaining. I have a feeling this randomness is part of what keeps us anglers coming back to this or that lake. Dalaran especially has a lot of added fun things you can find with Fishing to mix it up.
My personal favorite thing that I came across was the fountain, as is displayed in the video and previously here on WoW Insider. Various lore characters that have been in Dalaran at one point or another have tossed coins in the wishing well, and they come with quotes from the characters. Characters from the games, the novels, everything, they're there. It's really pretty cool.
The more random things are fun, too. The Hungry Tuskarr buff makes me giggle, and the Wild Magic adds a little "Hrmm, what will happen this time?" to the experience. If you're a Fishing fiend, I definitely recommend hopping over to El's Anglin' and taking a look around. The site has pretty much everything you could possibly want as an angler in WoW.