Even picking an expansion, TBC varied across its existence. Very very few people (one guild) ever ran the original original Magtheridon. The slightly nerfed version after that was also still a lot harder than the version most people knew.

Early TBC had a lot of pass/fail mechanics that required quite a bit of attention and you had to specifically build for. A really basic example is hydross. You needed two tanks in different elemental sets rotating and letting a debuff fall out. Sounds simple, but the old elemental resist thing, besides being a gear check, was also incredibly binary. If you had even a 1% chance of taking 100% damage you were kinda fucked. Had elemental resist worked like armor and you just reduced an amount of damage by 20-30% that would have made the fight way easier than the distribution of 100/75/50/25/0 damage. Late TBC actually had better gear checks like Brutalus that checked your whole raid's damage... something they already should inherently be wanting better... and your tanks survivability... something you already should be doing. So depending on what specific era you left off at it's kinda specific.

Similarly attunement is easier by far, and there is always something you can progress on. You can play casually and still see upgrades for a fair bit of time. You can play super-hardcore 5-man and aim for mythics and that's still progression.

There is vastly more movement/interrupt based skill checks in this game now. There's less obtuse uncommunicative mechanics. There's still a lot of issues with the game and choreographing movement/aoes/etc, but I think largely they've done what they could with the existing engine. What I mean by that is part of the difficulty of old dungeons used to be that the game just kinda sorta told you vaguely around where you would get destroyed, and between lag and animations you might get fucked despite "doing the right thing". There's less of that. Things that in wrath people solved with addons that were eventually disabled. Addons that literally just presented how god-damned simple it was to avoid standing in the fire, but the animations didn't do a good job of telling you.

Class homogenization may have taken away from some of the class fantasy of the game. Up to you how much that means. I literally was a player that started druid in vanilla because "faerie fire sounds useful" and that spell doesn't even exist. They also did things to gear that make it better but worse? It's kinda more RNG, but there's less loot competition and loot is more likely to at least be OK for you, but optimization is a bit weird with things like necks/rings that are way lower level but way better. Do you like playing mathcraft or not? It's easier now for a lot of specs, but not every single thing about it is easier. They applied a lot of Diablo III mechanics in this expansion for good or ill. I love Diablo III (for an entirely different reason than Diablo II), but there are definitely times I feel a little bit weird about it. Like most game mechanic changes, it's not better or worse necessarily but has different strengths and weaknesses.