I'm not sure how long you expect dungeons to stay fun when you plow through them every day.
Niuzao is a good example. Everyone in our guild really liked it. Some interesting mechanics, changes of scenery, it's well set up from the zone. There's even some humour.
But now we groan when we get it. No fault of Niuzao. It's just longer then some of the other options. The interesting mechanics lose their novelty. The humour gets old.
Another example. One of the things most people in our group commented on while leveling was the sniper quest in Jade Forest. Everybody thought it was fun ("hey, this should be a daily" was pretty common). If you stuck something like that in a dungeon it would probably be fun ... the first 10 times. Then most would gladly exchange it for a trash pack that could be aoe'd down.