I actually remember going back to naxx40 at one point in tbc with a 40man of T4+ geared people and some of the bosses were harder then level appropriate stuff.
I remember the group did it quite a few times and struggled quite a bit only getting past saphiron once or twice. while you can argue that they were not as mechanically complex they were hard for other reasons that were very unforgiving.
There were allot of reasons you couldn't just brute force your way through allot of the content back then regardless of item level as mentioned in a few posts above. you could never just be careless and pull as much as you wanted because you simply didn't have the tools to deal with it.