The sounds are very annoying, particularly the little screaming noise they make when they're exiting the level, and the release rate is all wrong – they're far too far apart for a 50 release rate in the first couple of levels.
Also, trying to integrate facebook is the most annoying thing ever in the entire world.
And on the level info/select screen, instead of "Click here to play", it should really be click anywhere to play. Mainly this is because my instinctive reaction is to click on the thumbnail of the level, and when this doesn't work I get confused.
Levels 4 and 5 are missing some terrain pieces, incidentally; the water on level 4 just cuts out in mid air...
And yeah, not being able to assign climber, floater and bomber during a fall is quite a serious omission and is
guaranteed not the way it is on the Windows version given that, as the others say, some levels such as Tricky 12 require you to assign floaters to lemmings falling out of the trapdoor. A famous Lemmings trick is to assign a floater close to the floor in order to speed up a fall (because floaters fall slower than fallers) but still be able to land safely.
As for selection criteria, the most basic one that you should get into the program is that
the lemming who was released most recently from the trapdoor is selected first – this will solve the Fun 9 problem for example. Another problem I found while doing Fun 9 is that I couldn't select a digger over the same spot as a blocker, but that's a bit of a niche problem, perhaps.
And as for the trapdoor, what happened to "Let's go!" and the trapdoor opening at the beginning of the level?
(But I think it's cool that you've done this, at least! I'm just being nitpicky really...)