In my Modern campaign, I have a martial artist who uses Improved Trip. In Modern, the feat doesn't give you the +4 bonus to the ability check, but there's a feat called Combat Throw that gives a +2 bonus to this check, and he has that feat, too. Finally, he has a feat called Improved Combat Throw that lets you trip an opponent who just missed you in melee combat - a feat that is balanced in Modern but would be horribly broken in DnD. You get lots of feats in Modern.
It's a very powerful technique. For starters, it's a non-FX campaign, so no monsters (virtually nothing Large with high Strength). The opponents are usually humans, occasionally animals, or
maybe remote-controlled robots - no Terminators. It's rare to run into an opponent with a +5 stat modifier to Strength or Dex (the PC has a +5 bonus on his Strength check when tripping) or an opponent specifically designed to resist being tripped.
But the real problem was the "trip loop". You trip an opponent, you get an AoO on them when they try to get up, then trip them again. Sometimes opponents would just give up and fight from prone - this happened to a SWAT team at one point. (The SWAT won, however.)
At one point I house-ruled that getting up from prone doesn't draw AoOs, and the PC stopped tripping, basically wasting two feats. Now I changed the house rule so getting up draws an AoO, but you can't use this AoO to trip the opponent. I've only had one combat since implementing the house rule, but for some reason the martial artist player avoided close combat that encounter (which he has
never done before), using grenades and even a whip. He only tripped one character, but got a crit on him when hurling him to the ground which killed him, so he never actually reached the point when the house rule would have come into effect.
Blargney the second said:
They don't have the various Improved feats and they're still willing to suck up the AoOs.
The only such trick I've seen PCs try to pull without the appropriate Improved feat is bull rushing. When your opponent is at the edge of a 5th-story building, the temptation can be irresistable.