Li Shenron said:
If the trip action is too good tactically, then it should be toned down.
But there's a million way to nerf something, and limiting it to 1/encounter (plus limiting it to only characters that have a special power) is just wrong to my ears. Much like forbidding to jump or grapple, if they were too powerful. I simply don't care if now is more balanced than before...
The idea is that if you want someone to do something every round no matter what class they are, you make it a basic combat action. Because if it IS a basic combat option SOME player will think its the best option and use it every round.
Unless you make it too weak to want to use it every round. Then no one uses it. There isn't a real middle ground.
If you think it is the sort of thing that only one class should do, you give it to that class as a power. If you can envision a member of that class using it as every attack against every monster you give it to them as an at will power(since, the rule applies that if it's at will SOMEONE will use it every attack).
If you envision it as something that, say a fighter type would use every once in a while, you make it a per encounter power. Which, I think, is the perfect place for a trip-like power.
The idea is the design the ruleset so that you get the results you want. This is rather different than the 3rd edition design of "Allow everyone to do everything you can possibly think of that they could do and make it a bad option for 90% of the people to do, then hope that players end up acting the way we hope they will."
Otherwise you have these options:
Trip is good for everyone - This is a good idea if you want wizards tripping your enemy fighters and ogres. If it is a good option in almost every circumstance then why attack an enemy with a normal attack?
Trip is only good for some classes - Why put it in the combat section if no wizard or warlock will ever attempt it? New players and even veteran players are going to read the WHOLE combat chapter or scan through the combat chapter on a regular basis when they look for rules. Why not make them short and filled entirely with options that apply to everyone?
Trip is not good for anyone except in special circumstances - Once again, why use up combat chapter space for something almost no one will use? Put the rules for the option into a power or feat so only those who want to use it need to read it. If it's a useless option for everyone else then don't even list it as an option to avoid distracting new players with options that might seem good but aren't.