Super-guild has already cleared Naxx

[Warlock]

For an easy instance there sure are a lot of people that never managed a single boss in BT. Adding Sunwell to the game didn't help keep a single subscription from that portion of the player base that's recruiting for Kara or Gruul. Things are generally undertuned at the moment - from solo outdoor mobs to Naxx-25 - but I do think a model of "most players get something each patch, we know the top ten percent will trounce the basic version but they'll have harder achievements to aim for" is better business sense than either BC's or classic's approach.