Some people love their mods, downloading every single new thing that comes across the pages of Curse or any of the other addon sites. Others are very minimalist, using only what they need to function for raiding or PvP. And yet still others try and completely avoid mods altogether. The last group is clearly the minority; the mod community is a huge part of WoW, and mods have become such a common part of the game that some people forget what the default UI looks like.
Throughout the game, Blizzard has seemingly had a love/hate relationship with the mod community. Some of the mods it felt were so functional and helpful that it absorbed some of their functionality into the default UI; the raid frames are now similar to Grid, your map lights up with objectives and arrows like Quest Helper. Others Blizzard felt were overpowered and changed the game in order to break them, such as the original, one-button Decursive from vanilla WoW, and recently, AVR.
When I was in the recent beta, Blizzard turned off the option for any mods for a while, and the game was a flood of QQ from people who basically claimed they couldn't play with the default UI and without their necessary mods. Personally always running mod-light, I adapted quickly to the mod-free play, and it even led me to dropping a few things from live, such as my damage meter and chat mod. I have found the game functions fairly well without mods, but I do still require my raid warnings, threat meter, and my guild's loot mod.
What mods can you not function without? Has Blizzard ever changed the game to make a mod you felt like you needed no longer work? How long did it take you to adapt to playing without the mod again? Or are you one of the few who refuses to use mods at all?