Was Naxx really that hard?

[secondary]

Simple answer, no.
The great problem with this dungeon was the high gear requirement which most guilds never met.
Clearing spider or abomination wing was fairly easy. It was even easier then clearing AQ entirely thats why most guilds who started Naxx progress didn't bother to try on C'Thun anymore as it simply was not worth it.

[Armilus]

Spiderwing and the first boss' of the deathknight wing and plague wing were about as hard as the first half of AQ40. The abom wing was about on par with the later half of AQ40 except C'Thun. That's as far as I got.

Aside from our tanks and caster dps, we didn't use a lot of consumables until Twin Emps in AQ40. I think Exiled will have a much easier time than my retail guild but it will still be challenging.

Naxx was the first instance that was really tuned to max raid DPS and healing. If you had 1-2 DPS or healers that had sub-par gear or didn't pay attention then you were screwed and most guilds weren't used to that.