Sometimes, your computer is from the stone age, and it can't run addons - unless you like lag that lasts forever. When you have a computer like that, it's best to run cold turkey. But for the rest of us, there are a lot of neat addons that can make things much, much easier. Or just downright more fun. These addons can be found at websites like WoW Interface, and at Curse Gaming.

Omen (WI)
Deadly Boss Mods (WI)
Recount (Curse)
Healbot (WI)
Decursive (WI)
Need to Know (WI)
Sexy Map (Curse).