For me, there are several great accomplishment like immersion, feeling of the enviroment, but having played Oblivion recently, the major accomplishment for me is that the game actually makes me feel like going into dungeons, forts, search for hidden places and explore insides places of the lands. While in Oblivion, i really didn't care for any dungeons, they were just not interesting and i never understood the point, so I went only when a quest sent me there.
About the major flaw, I think it's just unacceptable all the kind of issues there are with the port and bugs right now. Seriously, I'd rather the game delivered a few months after but have it well tested and fixed.
And after new vegas, I was hoping that finnally my decisions had some impact in the world, but atleast so far it's much more improved than Fallout 3, wich was an extreme let down storywise.