My old guild on retail before burning crusade was able to clear the Arachnid wing after two weeks of trying with the raid zone first rolled out. We never completed any of the other wings before the expansion hit.
That shit was hard. It's like this. Molten Core is a raid where you can take 25 people who are flasked and paying attention and the last 15 can be doing stupid shit like standing in the fire or going afk. It really doesn't matter how much they contribute.
Naxx was not like that at all. Our guild felt that there was a leeway for error with 5 peeps in 40 making minor mistakes or having slow reactions, with the other 35 being forced to be on top of things. At least for Arachnid wing. This was using a mix of BWL and AQ20 + quest reward gear w/ Ony drops for the attempts, so not best in slot but not horrid either.
I felt really, really proud of our team getting that far. Like the OP said, the raid wasn't seen by many people for a reason.