QQQ said:
It's pretty pointless to trip someone that is currently prone.
Why do you want to trip a prone character?
If by "prone" you simply mean "lying on the ground" then I agree with you, it is pretty pointless. If, however, as some folks feel, "prone" includes "standing up from prone", then it is indeed, quite advantageous to trip. My point was actually to show how silly the arguement is that if it's not explicitly denied in the rulebook you then should be able to do it. Common sense and reason must step in somewhere.
What happens in your game if unlucky drinks a healing potion? Has he drunk half the potion before the attack of opportunity occurs, getting half the healing?
Can Unlucky drink his potion while prone? If so, then he can drink the whole potion and gets the full benefits, even if he is tripped. The trip interrupts his action, but I don't see why he can't finish it.
Can Unlucky keep getting up from prone after he's been tripped while trying to get up from prone? That seems a little unreasonable. At least, not without using up another move action to so so.
If you think it is reasonable, then clearly, tripping someone who is using their move action to move 30 ft. through your threatened space allows them, paradoxially, to keep moving to the full extent of the 30 ft. mark after you successfully trip them as part of their original action. I haven't read any rules explicitly preventing this, but do you feel that this is within the spirit of the trip attack?
I disagree. A trip attacks alters the foes state to prone, nothing more. The tripping has no effect as the target was already prone. Does a successful AoO in your games stop the completion of drinking a potion when the target is in a prone state? IMHO a successful disarm/sunder AoO would stop the drinking but not any other successful attack (including tripping)
In my games, if you can perform whatever it was you were attempting to do while being tripped while standing or while prone, then the action is completed after the AOO triggered trip is resolved.
I think the combat rules have been balanced with the intent of being fun and fast rather than being realistic (e.g. Wizards always getting away with 5 feet steps to avoid AoO). Constant tripping is no fun. Being a Wizard and never being able to cast spells when engaged would not be that much fun either. Having a fair chance to be able to do something in combat is important.
Far from being unbalanced, there are still plenty of options! Wizards can cast defensively and thereby avoid AOO in the first place! Or they can perform the withdraw maneuver to safely move back out of the threatened space and then unload on their next turn. Combatants who are fully concealed (ie. invisibility) do not draw AOO in a threatened space. Creative use the battlefield may provide some form of cover, which negates AOO. Spreading out and switching to ranged attacks can turn Tripper into a pincushion. Levitate above the battlefield and your Wizard is safe from his trip attacks. Blind him so he can't perform AOO. Tumblers can potentially engage Tripper without suffering his AOO. Gang up on Tripper and grapple him; let's see him try to use that spiked chain from the bottom of a dogpile. Someone will get through, either by soaking up all his AOO or by having him fail on some of his trip attempts. Step out of his threatened space and charm him, and get him working on your side for a bit!
Having combats degenerate into tripfests is not the way to go. My paranoid self is still worried about the game becoming a grapplefest 
I think that invention of new states opens up to many cans of worms.
I fully agree with you on the states/worms thing. (That wasn't actually me you had quoted on that.) But as a DM, I'm far more worried about the party barbarian power attacking with her greataxe than I am of the monk trying to trip one of his opponents and keep him down. Far from every combat degenerating into a trip-fest, now people have viable and effective combat options other than using the biggest two-handed weapon available to max out their damage.
-- Vurt