I think part of the problem with TBC Classic, is…
When TBC first came out, it boasted several improvements to Vanilla WoW, it was more refined, quests were better designed, quest rewards were better designed, classes were better designed.
It was hailed as being awesome. And it was, at the time.
Then Wrath came out and made even more progress towards refining the game even more, removing annoying little tidbits that always rubbed people the wrong way, adding in new awesome innovations, etc.
Fast-forward until modern day, people went into Vanilla with the “Remember when?” mindset, and it was cool. But we knew that TBC Classic came, and then Wrath would be after that.
After seeing some of the stuff that’s missing that was introduced after Wrath, the whole pile of “I wish we had…” started to pile on. Every time you reminisced about “Man, I miss transmog…” or “I wish we had dual spec…” it started to make TBC Classic (and Vanilla Classic) feel that much less awesome and more “meh…”
I remember leveling my Ret Paladin in TBC Classic (my original character was a Ret Paladin, that I started in Vanilla, but stopped and went to a Warrior right before TBC came out, didn’t play said Ret until mid-Wrath) and I remember thinking just how terrible its design is. That judge seal judge seal thing was just the definition of pure tedium and I don’t get how anybody found that “fun”.
Wrath Prepatch drops and it goes from “bleeeeh” to “yaaaaay” overnight.
Now I look back on TBC Classic and it’s “Classic is way more fun now that we have Wrath”. A lot less features missing, a lot smoother gameplay, combat that actually works, etc.
TBC is the literal definition of Rose-Tinted Glasses. It was better than Vanilla, but yet going back to TBC Classic just makes you remember all the stuff you’re missing, the stuff you like about Retail WoW such as Transmog, Dual Spec, etc.