TBC is praised a lot by people on bigger servers in bigger guilds at the time. The raids were not all that difficult, as a lot of the problem was fielding the team, most of the bosses were pretty easy, but like most raids, one or two tricky ones or new mechanics. Sunwell easily the exception.
Now, if you were on a smaller server and in a smaller guild that got stuck in the Kara/Gruul limbo due to, small rosters, failed raid mergers etc. TBC was redundant until isle showed up. Small server suffered a lot during TBC and the end of Vanilla. A couple 25 man guilds per faction who probably came from the 40 man scene before so their rosters were mostly full for the entire expansion. You spent most your time praying you could find a group for 5 mans and actually finish it or get someone with half a brain in the group. And you ran Kara....a lot...like...a lot...
Enter Wrath which was the saving grace for small servers and guilds as 10 man raiding became a thing. The problem a lot of folks had is wrath upped the raiding challenge and a lot of the Vanilla TBC people who were heroes then started to get exposed for sucking or being dead weight carries in a larger group setting. (this is so often over looked in the 25 man and 40 man scene when people look back). I do find it interesting that people hail MoP as difficult content, when really MoP dungeons were a return to the Wrath dungeon style. Pull it all, AoE call it a day. Which even in wrath didn't start until the Ulduar gear jump/ToC gear jump. I see why wrath is hailed as number 1 for a lot of people, it made the game accessible to all group sizes and realm sizes, while still providing some of the most challenging content to date (Ulduar mostly with ICC close behind 25H Anub was also tough). While it did have some filler; Naxx (still an enjoyable raid.....DANCE!). Also, Wintersgrasp was a great addition for PvP people.
Overall mostly nostalgia or people being part of one of the groups that content fit in those eras, or they came from TBC and the Kara rerun to wrath where all doors were open. I miss the first iteration of LFD, when for a short period it was realm only, was a great recruiting tool.
Currently I find Legion fun, and getting better with most patches, while it like all expansions has some glaring sore thumb issues, the raids have been good, there is a lot of shit to do currently in game (despite it being redundant dailies, it provides a power progression), we are finally reaching a point where alts aren't a chore. Legion is looking up. Really the only weak expansion on a whole is WoD, and a lot of that has to do with the terrible story, over emphasis on garrisons, and the sadness that was shipyard (which I think we could see again in an updated form on Argus)