Great video. I think there really is a niche for a youtube content provider specific to multiboxing. I think it will become very big in WoW when players realize that they will clear all content very quickly and guilds will start split raiding. Plus the hardware demands are so light that multiboxing doesn't take much.

Here's my list of addons:
Atlas Classic: For loot, etc
Classic Aura Duration: So I know when to reapply rend, dots, debuffs, etc
Classic Cast Bars: So I can set my round robbin interrupt and catch NPC casts
Classic Codex: Best Questing Addon
DBM: Boss stuff
EZJunk: sells junk auto
Leatrix Plus: Speeds up looting and accepting/handing in quests
OneBag3: Giant Bag
Vendorprice: gives vendor prices
OmniCC: Cooldown counter
QuestlogEx: Extended Quest log, because the Classic one is awful
Classic spell activation: Lights up reactive spells like overpower
Corduroy: Coordinates for mouse on map and character
speedrunsplits: Gives level times
Clique: For healing through VFX and buffing