This suggests what might be bothering me about the Oblivion main quest ending.
The reversal comes at the end.
My intuition tells me that this is unusual. Most goal based stories--most video game stories--have a fairly reliable structure. They set the stakes, the win condition, and then they don't usual mess with it too much. They might raise the stakes, through villains or other opposition, but they usually don't make that goal totally irrelevant.
When they do, it's to make a point--it sets the other events of the story into a new light, that reveals new things. I'd list an example, but even mentioning that a movie has a reverse ending can destroy the viewing experience, because in the best one's you're not even expecting it, but it still makes perfect sense. Oedipus would be a good example, except that the audience is in on the secret from the beginning. (That's what makes it ironic, ya see.)
Oblivion's ending is not like this. It comes out of left field, is not anticipated at all, and doesn't cast any kind of new light on the existing events of the tale. It just makes the goal you thought you were working towards through the whole quest totally worthless.
The details of the reversal make it even more annoying. Basically, you find out there was a time limit on your goal, and you just blew it. Trick is, because it's a video game, you know that this "time limit" nonsense is ALL LIES. If the little number hasn't been counting down, you know it's just a cheap excuse for the game to spring it's reversal on you. No matter how fast you complete the quest, or how slowly, you're always going to show up just five minutes after you were supposed to.
It doesn't help that the NPCs act like they've known about the time limit all along. And by NPCs, I mean Martin. One of the neat things about the game is that it makes him actually useful, because he knows what's going on with all the daedra stuff. (And he's satisfyingly cryptic about just how he knows about it.) If you need to know something about the rules, how this whole "Oblivion vs. Tamriel" thing works, he tells you.
So when you suddenly learn this new rule, and he acts like it's totally obvious, but somehow never bothered to tell you about it, it's kind of irritating. Especially since, if I'd known about this rule, I probably would have picked up the pace a bit. Wouldn't have done any good, but at least I'd have known I tried.
I suppose the reason you're not informed of this secret time limit is because if you knew about it, it'd be totally obvious that it was going to come up. That's just how video games work.
However, if a plot twist is that obvious, the solution is not to withhold information from the player for no good reason. Make it some weird new thing that the bad guy's done, at the very least. Make the reason that the player doesn't have the information make sense.
Or, just don't mess with the twist ending. This is a personal quirk. I don't tend to appreciate twist endings, if they're done just for their own sake. They're usually either really obvious, or really stupid. Often, both. (Atlantis, I'm looking at you. First time I watched that movie, my brother had already seen it, so when I turned to him and said, "That guy totally goes evil and betrays them," about two minutes after the character was introduced, he got kind of weirded out.)
I'm also not sure if video games support twist endings all that well. That's another topic, though.
Bottom line: Don't totally change the rules of success for the protagonist at the last minute. Especially in a video game. It's irritating, and frustrating, and generally not a good idea. Like all "rules," in storytelling, you can break it, but only if you know what you're doing, and you have a good reason. Trying to trick the audience does not count as a good reason.