Originally Posted by Yora
But it all makes me wonder: Does anyone really enjoy such stories...?
Short answer: yes.
Okay, but digressing a bit more: it's actually pretty hard to diverge from the three-act structure. Not impossible, but as Saph says, the reason it's so common is because it's simple and functional. You see it in plays, you see it in trilogies, you see it in single-volume works even if it's not explicitly labeled as such.
The first act is pretty much inevitable: if your work has a conflict, you have to introduce it. It takes a very unusual story to dodge this step.
The second act is not essential, but it's smart to spend some time building up your problem, exploring it in detail and making it memorable. A problem that is introduced and promptly solved doesn't stick in the reader's memory.
And the third act is necessary to, you know, resolve the story. You can leave this part out, but it's a very unorthodox decision and will require careful handling not to annoy people. Even if you don't tie off all the loose ends, you want to reach some kind of closure, rather than just chopping off in the middle. Mass Effect 3 and Return of the King may be your least favorite parts of the series, but imagine that those trilogies had simply ended with part 2. No conclusion at all. Would that have been better?
(Actually - maybe you shouldn't answer that for Mass Effect 3. From what I hear, it)
I can't think of many stories that try this; the only one that comes to mind is the book Icehenge
. (Which I did like, but the ending was frustrating the first time I read it.)
I'm a little puzzled that you dislike the resolution step. At some point you have to, you know, resolve all those lovely plotlines and story elements you introduced in acts 1 and 2, or else there's not much point in having them. A story that constantly introduces new things and never brings any of them to a satisfactory conclusion would be... bizarre at best, unsatisfactory at worst. It's true that the resolution can be boring if everything that happens was telegraphed in advance, but I've never felt that way about Return of the King. (Can't speak for ME3, since I haven't played it.) If you can predict what's going to happen, that's the fault of the story in question, not an inherent flaw in the structure.
And it's not impossible to introduce new elements in Act 3! Looking back at Return of the King, there's the whole situation in Minas Tirith, which has been alluded to but not actually seen; there's Aragorn's quest for the army of the dead; there's the rescue at Cirith Ungol; there's the scouring of the Shire. (Yes, I know that's only in the books; I feel the books are a better example of a strong Act 3. The movie does an awful lot of foreshadowing and spends more time on battles than on scene-setting in Minas Tirith, Mordor, and the Shire.)
Personally, I feel like Act 2 is often the weakest one. The first installment in a trilogy is generally self-contained: it sets up the premise, introduces most of the major characters, and usually concludes as the protagonists reach a milestone of some sort. Sovereign is defeated, the Death Star is destroyed... you get the idea. But there's a distressing tendency for the second installment to be nothing but a prelude to the third, rather than a story in its own right. It moves the story along but does not have a proper conclusion, just a lead-in to part 3. Sometimes that works (The Empire Strikes Back and The Two Towers being classic examples), sometimes it... doesn't.