The people who are saying that Cata and WotLK are ‘similar’ are doing what people on these forums have always done. They’re not thinking about the games holistically, they’re just boiling the process down into a comparison of endgame mechanics and concluding that raiding in Wrath is not significantly different to raiding in Cata, save for a few tweaks and difficulty changes.
Because as afar as they’re concerned World of Warcraft is: “A game where I log in, run raids, get the best loot I can and do my best to get the highest possible parses while following the meta as closely as possible so as to achieve peak min/max performance.”
And for them it’s case closed - that defines the game for them.
So of course if you narrow you focus in such a strictly confined manner then obviously Wrath and Cata are gonna seem similar in most respects.
Take a step away though, widen your focus and try thinking of World of Warcraft as something other than “a raiding experience” and it soon becomes clear that most of the key differences between Wrath and Cata are based upon the total and complete remake of the old world, and the destruction of all pre-Cata questing/levelling content.
We have a much more streamlined, faster, linear, super-convenient experience where almost all roadblocks and bottlenecks are removed, many or most forms of travel and exploration have been made obsolete or removed. ‘Inconvenient’ mechanics such as reagents, poisons, soul shards, ammo etc. have been removed, quests made much simpler and more streamlined with massive XP boosts for super-fast levelling group content has vanished and other players are no longer a requirement to complete your journey - not to mention the fact that world mobs are significantly nerfed, and players significantly buffed so that the slow cautious approach of old no longer applies and we can now just steamroller over quest mobs without even the need to slow down, let alone engage our brains.
This is the real reason why Classic era ended with Wrath. It has little to do with end game, or even PvP and everything to do with the journey to get there. A journey that Cata no longer regards as important.
Yes, Wrath paved the way for some of these changes. The expansion put some of the building blocks in place - but it was Cata that truly changed the game to something entirely different.
If you “only care about end game”, than all of the above will mean nothing to you - but if you are anything like me, you’ll understand why Cata was so universally disliked by many of the more oldschool MMORPG players.