I was thinking maybe it's because they need 2 kicks in order to wake up--one in the level above them, one in the level they're in--because of the sedative that they used. I mean, they had to sync up a kick for every level, so that's the only thing I can assume would be it.

i actually thought that hitting the water was the only kick.

heres how i see it.
to come back from the city: Timer goes off
to come back from the hotel: Hit the water
to come back from the snow: Arthur's explosion
to come back from limbo: the defibrillator

now i could be wrong on all this but if going through the guard rail was going to be enough to wake them up then the van doing a barrel roll would have done it as well.

Yet it was specifically mentioned that that was the first kick, and they missed it. The car hitting the water was the second kick, presumably as a back up in case for some reason the first kick didn't work.

when the van goes into free fall, everything in the hotel dream goes weightless. now since they are weightless, wouldnt they go weightless in the snow level as well?

maybe events like that only go one dream deep. otherwise you would only needs one kick to wake up out of any number of levels.

This theory makes a good bit of sense, yea. I'll run with it.

So I just saw it. Sweet movie. Also about the top falling: It falls the first time (or second time) we see it when Joseph Gordon-Levitt's character comes into the hotel room after the first "job" right before they go to the roof to get in the heli and asks if he's okay and then again in the bathroom after the "testing " of the strong sedative when the Asian guy comes in and asks roughly the same question (it falls on the ground and stops spinning). He was in reality for at least a part of the movie IMO.