Heh funny thing about video games, no matter how much content you put into them they have to end sometime, of course Bethesda did try and extend the life of the game by having random repeatable quests but lets face it they get old fast. I guess the only thing left is to either try out some of the player made mods, make some yourself or wait until Bethesda releases some Skyrim DLC, that is unless you are on a console in which the only thing left is to wait until Bethesda releases some Skyrim DLC.
Though if I may ask, how many hours did you manage to rack up? Also has there been any word on DLC yet?
And, as people have pointed out, the OP would have had to spend at least that much time to get to level 51. And it's highly unlikely that he's discovered close to I'm-going-to-guess-and-say-half of the game world.
Well to be fair considering that most quests follow a pretty linear cookie cutter design (find this item in the end chest in some random dungeon) and are only really there to pad out the length of the game one doesnt really need to see everything in the game to get the gist of it, I mean I know this game boasts over 300 hours but considering most of it is cookie cutter crap I am sure the OP isnt missing much by not doing a few fetch quests.