I only started seriously raiding in Wrath, and didn't even realize Naxx was the same as before untill we were fighing Yogg. I really liked fighting against Onyxia. In Cata, I liked revisiting Deadmines and Shadowfang Keep. I've never seriously raided those two Zul's either, and I look forward to exploring those places.
I personally have no problem whatsoever with Blizzar re-using some older content. These places are so well known partially because they were so well designed. They are good dungeons with good fights. If those dungeons were designed four years ago or two months ago doesn't really matter, quality does.
There's a third type of player that I've encountered as well. These are people that did all these instances when they were brand new, and they take great pride in telling people how much more fun and challenging they were. If I have to judge this from how often they repeat it, I conclude that they enjoy saying this.