One of the more interesting parts of WoW tooltips, in my opinion, is flavor text. It's not found on many items, but that yellow text occasionally gives me a little chuckle. Plus, the developers are known for putting yellow flavor text on spells, as well, especially spells that players will never see. Some examples of my favorites:
The Fist of Subtlety from a quest in Zul'drak, which simply reads: "Not at all." I confess to laughing way too long upon reading this one, and wishing I had a class that could use fist weapons.
Extinguish All Life, Yogg-Saron's enrage timer. At 15 minutes, he simply wipes the raid. Not fair at all. However, it does do almost exactly what it says: "Extinguishes all life. Kaput."
However, Yoggy pales in comparison to Archimonde, who - if he can't reach anybody to smack them around - starts giving everyone the finger. Specifically, the Finger of Death, which (in addition to being a nod to D&D) "strikes an enemy with the finger of death, inflicting 20000 shadow damage upon them, their children, and their children's children."
Those are just a few of my favorite examples - what're yours?