A frustrating movie... (spoilers)

I haven't watched a movie in a long time which gave me such contradictory feelings. I mean, I actually, really liked the movie. It was cool, it really was - there were so many subtleties, in the settings especially, that I found fascinating - but the movie itself seemed brøken. The idea of the movie seemed simple enough to think it must have been done before (and better), but I can't think of any examples. If someone can think of a better version of this movie, I'd like to see it.

Now, to my frustrations:
1) The makers of this movie went through great pains to keep you from knowing exactly what was going on for as long as possible. This is reasonable. Unfortunately, they decided it would be prudent to delay the whole concept of the movie (evil dopplegangers coming out of mirrors that kill and replace people) for 50 minutes (rough guess) of the 80-minute movie to keep the suspense. All they did was replace events with that low, forboding noise that slowly gets louder, building suspense to foreshadow some coming climax... (I don't know what proper name this would go by)... to have nothing happen. Perhaps those were the times the dopplegangers were supposed to emerge from the mirrors, or something; either way, it was pointless for the watcher - after the third time the sound game and left and nothing happened I felt cheated, by the fourth it was gimmicky, and by the seventh...

2) OK, so 50 minutes passes by, and they decide to let the real plot of the movie be shown. Dopplegangers killing people. Kate gets brutally killed by one (and you see they are rather superhuman - I couldn't imagine the force required to punch your fist all the way through someone's teeth, through their mouth, and into their throat. How the hell she made that much noise in that situation, I don't know...) Well, they had to give it away eventually; they may as well do it in grand style, right?

3) My big complaint with the movie is this: Usually when someone feels they don't understand a movie, it's because they don't understand What happened. The What is often withheld from the viewer to good effect - it leaves it up the person watching to finish the story by himself, and wonder what happened. But we know well What happened: dopplegangers killed off and replaced people in this family, including the main character. Instead, this movie withheld the How and the Why. Since they keep you from seeing the What, as well, for the first two-thirds of the movie, the movie seems empty for a long time. Suddenly the whole movie happens in the last 30 minutes. It's over. They don't explain How these dopplegangers are (in the mirror, on the other side? coming through? impressive but inconsistent in their imitations? causing mirrors to fall? choosing their victims? related to the goings-on of the family the movie centers around?). Even more frustratingly to me, they withhold the Why: especially, Why are the dopplegangers coming out to kill this family now? Because of seven years of bad sex? No character is shown breaking a mirror in this movie - they all get brøken by themselves. Sheesh. What's the deal?

Not knowing What leaves you in thoughtful imagination. Not knowing How or Why leaves you empty - it becomes a movie without a point.

For all that, I think this movie had great vision, the idea was rather cool, the acting all ranged between more-than-adequate and perfect, etc. I liked it... but man, did I leave feeling frustated...