Let's start off with what I see as a pretty massive plot hole.
If MIB's fake-mother knocked him out in the well before he put the wheel in the wall, killed all of his people, filled the well and left him lying outside, how is the Frozen Donkey Wheel even possible, considering it was never put in the wall? Can we assume there are donkey wheels everywhere MIB's people found the electro-magnetic-magic-light-pockets? If this is the case, why didn't he just turn one of them and leave?
Also, the smoke-monster explanation is a little frustrating. If there was ever going to be a time to tell us what the smoke actually is, Across The Sea was probably a good time. Hopefully we'll get an explanation of WHAT the smoke is, where it came from, how it came to be etc. At the moment, I'm still left scratching my head on this one.
Also, the initial light-cave near the water made me cringe. Probably the cheesiest thing I have seen on LOST so far. And those kids made me want to hit things. Punks!
ALSO, I would have much rather seen Adam & Eve be a couple that we've known a bit longer than one episode. It would have been cool to be able to rewatch season 1 and say "YEAH, THAT'S TOTALLY ROSE AND BERNARD JACK! OMFG!" but now we have to say "Yeah, those two people don't really mean anything to you Jack. Move along."
I seem to be one of the few who was disappointed with this episode, after a string of decent ones. Here's hoping the next episode and the finale deliver.