A few things that haven't been mentioned.
AoOs are resolved before the action that provoked them. For instance, if you cast a spell without casting defensively and while standing in an opponents threatened square, you provoke an AoO. If successful, you may lose the spell. Thus the AoO is resolved before the action that provoked it is resolved.
With standing up from prone, yes, it provokes an AoO, and that could be used to trip an opponent. However, the act of standing up isn't resolved until the AoO is resolved. Ergo I start to get up, you trip me and then I resolve my action and finish standing up. No unlimited trip, attack, AoO trip, attack sequences allowed. Using trip attacking an prone opponent is a waste of an attack.
Also, tripping is an unarmed attack unless you have a weapon specifically designed to aid in tripping. Of the reach weapons, only 3 core weapons allow this: spiked chain, whip and guisarme. So the longspear example given earlier doesn't work.
So yes, tripping is much more powerful than it was in 3.0. However, it still takes 2-4 feats to do this effectively (Combat Expertise, Combat Reflexes, Imp. Trip and EWP, Spiked Chain), you do risk being countertriped on a failed attempt, and many oppenents are going to be very resistant to this attack (large enemies, enemies with more than 2 legs, those pesky dwarves, etc.).