I'll have to reconsider and double check the rules as is, but at present, I'm inclined to think that being able to control an area and prevent people from moving past you without it even being your turn is a bit game breaking.  I certainly don't have any objection to your character readying an action to do the same though.  (which is about what a spellcaster would have to go through, with the spellcaster taking the additional burden of provoking attacks of opportunity, counterspells, antimagic zones, limited uses per day, etc.  Though to be fair, the spellcaster will usually cover a larger area and have a slightly better chance of affecting foes with their control spell.)<br/><br/>I don't mind you respeccing if you think that not being able to trip attack during an AoO makes improved trip a dud feat choice though.  (though it's my rather strong conviction that it doesn't.)