With the Classic panel it was brought up that Blizzard wasn't sure what to do with add-ons. So, it would be a good time to discuss what add ons would and wouldn't be good to have.
Now obviously, anything that was banned or purposely broken at any point wouldn't be included. UI ones I don't see any problem with, unless there is something I'm missing on those. Definitely want one to fix enemy health bars fighting for your screen.
Blizzard did mention ones that played your class for you. I'm not aware of any personally unless I'm unaware of Blizzard's definition of what qualifys as one.
I see DBM and equivalents being a hot debate. That one I'm not sure of.
I didn't start playing until Wrath. So, my knowledge of Vanilla add ons is limited. What add ons do you remember being very good or being too good to have? What about modern ones?