Bethesda's mission statement with the TES series is supposed to always have been:
- To create a wide-open, no restrictions world in which "if you see something, you can go there"
- To always begin anew, recreating the franchise with each iteration
- To push the boundaries of what's possible with a CRPG every time
Now, I think it's safe to say that they have accomplished relative success in each these goals with TES I - IV, yet obviously to varying degrees of competency and in width of reception.
Arena was by all means a groundbreaking game. It featured, IIRC, the largest explorable world to date in the industry, and blew people away with its open endedness.
Daggerfall was a rousing success, following successfully on Arena's formula, and had its story and lore fleshed out significantly.
Morrowind started pushing the technical aspects more, with a smaller gameworld, and fell short of many of its goals (see Vivec city). Still a great game, though.
Oblivion was Bethesda's Windows Vista. Pushed the tech quite a bit, but pissed everyone off in the process of losing its vision.
I think TES V might be a big surprise to many. As has been pointed out, Fallout 3 made improvements. You can make arguments as to its quality as an RPG, but this was their first venture into a serious non-fantasy and non-TES roleplaying game. I think they pulled it off well, and they're going to focus a lot of the things they learned with it (and with their former titles) back into what they do (did) best: get your sword, get your spells, we're goin' TES'ing again.
I look forward to it. Yes, I'm a fanboy, I admit it, but I think I'm also an objective one. If they combine the best of Daggerall and its huge, open world and better sandboxeyness along with Morrowind's quirkyness along with Oblivion's better tech and Fallout 3's polishing, how bad could it be?
On second thought, Codex, don't answer that.