Maybe he is referring to verisimilitude/simulation issues like:
Halfling rogue can use trick strike to trip up a gelatinous cube and slide it 4 squares. Or use bait and switch to change places with the cube that's taking up the whole hallway.
It is easier to push a Gargantuan dragon, than to push a dwarf.
I've seen and heard a number of players/DM's turned off by such mechanics. I say if it bothers you, use the DM trump card and get on with it, but that's just me.
+ you forget how to do a power by swapping it at higher levels.