My guild only made it to AQ 40/ BWL before TBC came out, and I remember the horrified stories about how much frost resistance you must have unbuffed to even think about Naxx, not to mention the insane rep grind with arget dawn/Scarlet crusade.
I do however think that the wotlk Naxx is fine as it is, mainly because the gear that drops there isnt rewarding enough (if you're gonna make naxx harder, then you should also make the drops better), and because most players (i'd dare say 90% of the players) doesnt have a clue about the tactics of Naxx, and generally just wins cause of their overwhelming gearscore that pretty much makes Spank-n-Tank possible on most bosses. However even with a raid of 5k gs people, wipes still occure quite a lot.
If you make Naxx a "must" for players to complete, instead of a achievment run which it is today, then I fear wotlk will have too many endgame instances, it can sometimes be hard enough to find raids for TOC, Onyx, ICC, RS for a lot of people. People who has achieved the GS to do FoS & PoS, or even RDF HC will never look back for Naxx, thus it will be increasingly harder for newcomers to find people who will even bother with completing that challenging Naxx.
Summary: I vote No. Keep Naxx as it is, and if you should make it harder / make it Vanilla, then do the gear drops slighty better as well.