TBC is the single greatest expansion in the history of WoW. It revolutionized the game. It kept true to many old concepts from Vanilla. It improved upon many less than ideal game play experiences, and it brought about an unprecedented level of subscriptions.
That being said, it was also the beginning of the downward spiral that has led us to BFA. Vanilla was engaging to players because it was this hugely vast world that was open and full of secrets to explore. It was so massive that even after 2+ years there was secrets still hidden that players had never discovered. It was dangerous. TBC was much the same. However it also brought about changes that, at the time, were widely heralded as massive improvements. Yet over time those championed changes were what has driven away much of the player base.
Flying mounts, while beloved by many, diminished the size and danger of the world by allowing players to fly above and away from the world at 280 or even 310% speed. Jettisoning from raid to raid on a PvP server without seeing so much as a single mob or cross faction player made the world feel small, tame, and less immersive.
The introduction of smaller raids lowered the need for large guilds and large, yet tightly knit communities. Why have 100+ people in your guild to fill 2 raid teams with a few alternates when you could have 30 people fill an entire guild and use their alts to fill raids to gear out mains? Why bring along 40 players (half of which were probably not very good anyway) when you could find 24 solid players and run weekly raids without anyone else’s help?
Heroic dungeons. Heroic ****ing dungeons. People loved them, but their daily boss quests had a direct contribution to the speed clearing nonsense you see in mythic+ today.
Arenas. Many many many players were so incredibly excited for arenas. Finally a level playing field where you and 1-4 friends could form a team, make a name, queue up, and prove your worth against other less fortunate souls. When in reality TBC arenas was all about bringing the best meta team composition. It was about who could pillar jump the best. It was eventually led to the destruction of class identity due to people not wanting to see the ‘meta comps’ running the show.
Raid and dungeon bosses began to get more and more mechanics for each fight in TBC. This was welcomed because by the end of pre-TBC there was groups of 5 people defeating Onyxia, a 40 man raid. However this too has lead us to the current queen azshara mythic raid with (how many???) so many mechanics in the fight you damn near need an encyclopedia opened telling you what to do.
TBC was a game breaking, phenomenal expansion that ushered in a new era of WoW. For many it was some of the best time they ever had while playing WoW. However it was also the beginning of the end. That being said, I personally think the safest bet for Blizzard when Classic is hitting a content drought after Naxx, is to release TBC. Any classic+ content simply has too large of a chance for failure.