Anyone can get bull-rushed off a 200 foot cliff, and damage from the fall won't vary much, regardless of level.
External factors should be balanced with other external factors, not at-wills.
Dropping a 20 lb. rock with mage hand (from the side) on someone is not much different from dropping a 20 lb. rock you have in your hands, without getting to look directly down on them.
The factors of damage and such shouldn't take powers into consideration.
Sure, you may be using different stats, but a falling rock is a falling rock.
In regards to your cliff example, I actually gave a specific mention to encouraging players doing things on the fly in response to the environment that are within reason. I have no problem with the cliff, because it is a neutral hazard. A player can bullrush a monster off a cliff (or other height), but a monster can do the same thing to the player.
If a DM allows players to make a habit of leading enemies to heights just to bullrush them off, without enemies making attempts to do the same to the players, then that DM is being a poor DM.
It's my belief that when players attempt creative actions to make the game more vibrant, exciting, and entertaining, then by all means, allow them a reasonable bonus or advantage to them, but do so in moderation or within reason.
What I am trying to discourage is people who are trying to plan and scheme of tactics that they can use to give them unbalanced advantages that they repeatedly use over their enemies, just because the DM will let them get away with it. I'll let them have their shticks, but I'm not going to let them carry around gallons of oil all the time and have them successfully use it to deal the equivalent damage of powers 5 of 10 levels higher than their level all the time.
Giving a player an overpowered at-will ability isn't a good idea.