I love the game.
I've had a lot of time to think about it, and I'm still not sure what I think of the ending.
I realize that, built into the story, you can't know whether ringing the six bells actually did anything. But I've always felt like one thing *should* happen: The Believers should think you accomplished something. In theory, ringing the sixth bell could reasonably lead to some kind of thanks or ceremony from them.
I agree that the actual ending feels anti-climactic. You don't even get to perform the *action* of ringing the sixth bell before you are cut off. It's certainly a deliberate design decision. And it seems, in part, intended to tell you "There was no need to rush through these bells. You've still got rooms to explore, and stories to uncover, etc."
So, rather than clap your hands happily, fulfilled by a thoroughly satisfying ending (say, like the thank-you ceremony at the end of the original Star Wars), you get a feeling that maybe you're not quite "done" with the game.
The game mechanics aren't exactly set up to deliver a scene where the Believers come to you, congregate around you, honor you in a ceremony, etc. And, for some reason, as explained by dialogue in the game, they aren't even especially concerned that you will be successful with your effort. So there are a couple reasons why the ending I would have preferred to see (maybe after completing all the rooms) wouldn't quite be right.
I love the game. I've played through it three times. And who knows? Maybe if it had an ending as satisfying as the ending to Uncharted 2, I wouldn't still think of it as a world I can return to and play some more.
I too wish that there were a sequel or more levels, although I'm not sure that a fan-made levels would do it for me, since I feel like the story-telling has to come from Nifflas himself. Maybe one day, and will think of some new stories he wants to tell, then crowdsource some of the puzzle design to others, and we can have the best of all worlds. I get more stories, you get more puzzles, Nifflas doesn't have to single-handedly do it all himself, but he does retain creative control, and so on. :-)