The perfect open-world sandbox RPG. It may have its glitches and quirks, but that is part of what makes the game what it is: A closed system with a set of given rules that you can ♥♥♥♥ around in to your heart's content, exploiting exploits at the corners of logic, and having a blast. Want to roam the wilderness robbing tombs and shipwrecks? Go ahead. Want to put on a cloak of constant levitation and float about taunting people on the ground? Go ahead. Want to kill a god (or two)? Go ahead. Want to follow the story (you weirdo)? Go ahead.
So that covers the sandboxiness of it. The atmosphere is another incredible part- awesome unique environments, incredible music (that Jeremy Soule basically just copied for the next 10 years for sequels), and memorable locales (literally- I can navigate Balmora with my eyes closed).
Some may consider the graphics to be subpar today (I don't mind them, personally), but there are loads of mods to fix that. Some complain about the click-click-click combat, but again, once you understand the system, just play within it.
So yeah, if you haven't played this game, buy it now. Even at $20. NOW. DO IT. And if you say you prefer Oblivion, I will cut you.