And then, once I got to about level 30, my friend Mark stepped in and told me about an addon called QuestHelper. My questing (I didn't do dungeons at the time) was revolutionized. I had navigation, routes to work the quests as efficiently as possible, and a plethora of other minor features that I don't recall. I do recall, however, that they were helpful! Mark told me that our mutual friend Paul didn't use QuestHelper, that he was a purist, and I couldn't for one second understand why someone would shy away from using such an amazing addon.
Fast forward a few years, and I don't have QuestHelper installed any more, but as I'm leveling DPS characters, I'm considering getting it again -- the dungeon queues are so long! Now, my addons are legion, competing for RAM with the game. So choosing one addon that I couldn't play without was seriously difficult. Do I go for TukUI, the UI replacement I use? Hmm, maybe -- but it's mostly cosmetic, hardly vital despite its handy features. Gladius is a massive help in Arena -- I really suspect I'd miss it -- but it could perhaps be replaced by some cunning work with macros and the like and the basic frames.
If I were to lose an addon, which has happened a few times through Cataclysm where they've been incompatible with patches, the loss that would cause me most bother is Bartender. It has all my keybinds stored up and saved in profiles, and it's ridiculously easy to set up and alter. For healing in the beta, I've felt the lack of Healbot, but I've managed. WeakAuras might be the one I've missed the most, for any other restoration shaman out there, this is a great way of tracking your Tidal Waves buff. I feel a bit lost without it.
Is there an addon you couldn't play without? Are you like my friend Paul who doesn't really go for addons? He doesn't even have DBM ...