A PC who can knock mobs prone or force their movement can wreck an encounter singlehandedly.
So...thinking of ways to get around this, other than the bad idea of completely neutralizing the power by giving the mobs a cheesy "immune to forced movement" attribute, what are some ways to make a mob more dangerous and/or punish the PC who knocks it down or slides it around the battlefield?
I don't want to completely eliminate the controller's ability to shove the mobs around, just make them think twice about it and have to make a hard choice.