Another website gave some kind of explanation as to why Chandler's no longer there (besides the fact that the dude got busted for possesion IRL), but it goes along the lines of the producers wanting to focus more on the Camdens, unlike seasons past.

I'm really glad that Simon doesn't look too much like an alien anymore.

Sam and David, IMO, still have a speech problem, if they're supposedly FIVE already, why are they still speaking like that? When the Olsen twins were 3, they were nothing like that.

Shut up, Nicole. Haha