I think it's a pure nostalgia which is why people are making the comparisons between the current iteration and Vanilla / TBC wow.
The game due to customer demand has turned into a pure loot piniata where bosses are defeated within a week of being released on live (after countless weeks on the PTR) and the game in my opinion no longer has the "elitism" of the old days. Everyone is decked out in epic gear without having to try whilst in Vanilla and TBC, people in greens were normal with rare items exactly that and the purplez were reserved for the best of the best. Hell I remember completing Benediction and Thok'delar with a degree of fondness despite the quests and tasks not being something that you could zerg to kill.
Most of the multiboss raid instances in TBC took a week per boss on average to clear (e.g. SSC took around six weeks before Lady Vashj was killed) and that was due partly due to tuning (some such as Kael'thas were unkillable until nerfed) but also because no-one knew the mechanics and strategies to beat the encounters were developed on live. In BFA those strategies were developed on the PTR and beta and so it's simply adjust to the minor changes and execute.
The game has changed as Blizzard have adjusted to the desires of their customers and for those that look back on the glory days it's just not the same. Classic WOW will always hold some appeal but I think the nostalgia factor is what is making people think it was better than what it was.
Also what Storm said, Naxx 40 was an experience and too short lived IMO.