If you have mounted combat, then trip does not affect you. Else, I'd say think of it as you ride up to where the other guy is, then dismount while you fight, then when one of you flees you both hop back on your horses. 
Briars of Pain
What about me flying? Or me riding a pony! My pony is as fast as the wind and no briar can stop it (whatever a briar means) 
If this were ever to be implemented I'd be satisfied with briar being a stationary entangle that hits someone every time they walk into it for ~2-3 hours.
I actually do not like it.
But the best way to code it would be to make a new invisible trap. Then trap the exits.
Also there are other hazards already implemented like the:
Tree fall north of Val Miran
The fall near HEralds cabal.
The Swans plunge in the Underdark
it was a suggestion, a small one to prove that I want to contribute more to this than simply playing it.
rolling a stone giant ranger <-----points to himself, stone giants just crush the weeds right? I mean, they are rock, I think :/
I suck so bad with rangers, dunno why cuz they are so good on paper. Polk you should try a rogue, seriously. Ninjas and Thieves are what always bring me back to FL when I leave.
Stone giants are not rock just as fire giants are not made of fire.
But are Elves still made out of ice-cream and sugar candy right?
Elves are short things with long ears made from a vortex of magic, and don't need wands like every other witch / warlock.
coughsGOLEM!coughs
And... all respect for Jibber is officially gone. I however, being the manliest of men, had to google around to find out that was a Harry Potter reference. Next I'm gonna go lift weights and eat a raw steak... RAWR!!!
As an effect in new area(s), it would certainly be sweet. A "random" trip when you move would be easiest to produce?