I suppose I should have followed through completely. The attacker can follow and allow for additional attacks. We don't play with many catwalks or cliffs (yet!), so that hasn't been completely a problem. What I would imagine would be something along the line of Reflex to grab a ledge or a Tumble, Jump, whatever, to avoid a too-quick demise.
Even with that, there's the same problem, though not as severe.
Push them to a cliff, and at most they're hanging on for their lives with no way to resist.
Push them towards a wall, boom, automatically prone with
no way to resist.
This house rule makes a full attack more effective at bull rushing and tripping than bull rushing and tripping are. And it gives the defender no viable way to resist it.
My two biggest problems with this, however:
1) Gnome vs Dragon. Gnome makes a full attack, and pushes the Colossal Dragon back 20 feet.
This is extreme, of course, but how about Halfling VS Dwarf? Or commoner vs level 20 Paladin?
An attacker shouldn't always be able to push an opponent back.
2) When fighting, if skilled at doing so, moving back is almost never forced, unless you're knocked over. Rather, moving back from an attack is a conscious defensive choice. (It can also be due to fear, but that's another story.)
As such, the defending creature should never be forced to back up (or fall prone). He should be able to stand his ground if he chooses, and should gain some defensive benefit from moving back if doing so.
Now, after that rant, I'm actually going to be helpful here. Try the following:
New combat option: Soften Blow
When a creature would be hit by a melee attack, he may choose to make a reflex save, with a DC equal to the attack roll. If he succeeds on this roll, he gains DR 2/- against the attack. (This DR is applied before, and stacks with, other DR).
Whether the roll succeeds or not, the defending creature moves directly backwards 5 feet. The attacker may follow as a free action.
I haven't decided yet whether I'd have this movement count against their movement for the round. It should be good either way.