What is there to do at Level 50? A lot!!!
What fun things are there to do at Level 50? Nothing!

Well, I guess it depends on your definition of "fun". If you're not into raiding and crafting, then you probably won't have much more fun with the game. Besides those 2 things, most of the Level 50+ content is just grinding (pretty boring).

By the way, you make it seem like "hard mode" dungeons are the exact same dungeon with the same maps and the same monsters doing the same attacks, as if they're simply boosted in stats and numbers and such. If you think that is the case then you are mistaken. The "hard mode" dungeons are completely different from their original counterparts. You should think of them as completely new dungeons. They are! Really.