As far as Naxxramas changes, our Lead Designer talked about this a bit in the past, a fight like Thaddius will be pretty much the same where a fight like the Four Horsemen will need a fair amount of changes.

[...] Think of it like this, if a 40 person fight requires numerous people to be continuously rotating around and sharing debuffs and whatnot, when we remove 15 people from the raid group, we can't require the same number of people to rotate, so it has to be updated to fit accordingly.

[...] Naxxramas has a pretty good setup with the 4 separate wings so you are choosing which 3-4 bosses to attempt at a time. You can also just try and kill the first one of each wing, whatever your group desires really. Yes, you have to clear all 4 wings to get to the last couple of bosses, but usually by the time you are ready for them, clearing them isn't a really big deal.