Oblivion? Without mods, not very good. It's a game with an uninspiring story, fair-to-good combat, very shiny but very bad aesthetically, especially when you keep in mind that Cyrodiil was supposed to be a vast jungle (and we know Bethesda can do great aesthetics because that was a major reason Morrowind was so great), terrible voice acting, terrible dialogue, terrible writing in general, boring and insipid quests, lack of choices, completely inconsequential player actions, AI which is so legendarily retarded that how they thought it a good idea to hype the quality of the AI is beyond me. Skills are simplified from Morrowind, itself simplified from Daggerfall, and I presume in TESV you'll have a choice of four skills; fight, magic, move-sneaky, and talk.

Don't even get me started on scaled levelling.

With mods - and I mean a lot of mods, FCOM is just the start, I currently have 100+ - it becomes a reasonably good game, but still suffers from quite a few of the above. I prefer Morrowind, at least that was an interesting and engrossing world.