The most anti-social addons are things like Gearscore or various damage and performance meter addons. I wouldn't mind if SWTOR had a built-in functionality which would somehow show my performance, but only to myself, with the goal to improve myself. But in WoW these addons were primarily used to point a finger at people who for some reason had a slightly lower level of gear or damage output. They were exclusion devices, with people organizing raids asking for "minimum Gearscore 6785", which happened to be exactly their GS, so they could be carried by people with better gear. These ePeen measuring devices in my opinion did more harm than good to the community of World of Warcraft.
Another type of addon commonly seen is the performance-enhancing addon. A healing addon like Healbot makes group/raid healing a lot easier than the standard interface. And addons like Deadly Boss Mods give advance warning of incoming special abilities from boss mobs, which make the "dance" a lot easier. So what could be wrong with that? What is wrong is that by making these functionalities dependent on a third party addon, you can't be sure that everybody is at exactly the same level. Performance-enhancing addons are like performance-enhancing drugs in sports: You might consider them "fair" if you assume that everybody uses them, but as soon as you consider that some people for some reasons don't use them, it becomes obvious how distorting they are. If Healbot is strictly necessary to beat an encounter, then why isn't it part of the standard user interface? Performance-enhancing addons make balancing encounters very difficult, because developers will have to decide whether they balance the encounter with or without the assumption that these addons are used. And if you balance the encounter assuming that everybody has the addons, then you'll get the occasional case where somebody is unable to beat the encounter because for some reason he didn't have the addon he thought he had. Again, if an addon was necessary for an encounter, it should be part of the standard user interface, not a third-party addon. Fortunately Blizzard at least put legal measures in place which prevent people from selling their addons. Imagine there was a must-have raid addon that you had to pay an extra $20 to get.
The least harmful addons are those which allow players to customize the user interface. But frankly, in this day and age complicated games should have customization options for the user interface built in from the start. I don't see the advantage of doing that with addons. In the end all addons are just crutches for functions that the developers were either too lazy to build into the game itself, or for functions where the developers thought that the game shouldn't have them in the first place. Blizzard made theirs far too powerful, leading to addons that you can't perform without, and others which are mostly used to stroke ePeens and separate the community into "us" and "them". In the SWTOR beta somebody was joking "6K GS required" in general chat when he was looking for a group for a low-level dungeon. I so hope that this remains a joke and doesn't turn into reality in Star Wars: The Old Republic.