Thieves:
If flanking a character engaged by a fighter, they can backstab constantly, because you can't take your eyes off the fighter!
Saves vs. traps or poison: Thief gets to add his dex bonus or roll twice, his choice beforehand.
Fighters:
Fighters are meant to wade into groups and be surrounded and still be a viable threat—how?
On a nat 20, a fighter does damage to enemies within adjacent squares to him. This reflects how monsters may try to take him out and his assumed role as a frontline man.
Hey, make up some of your own, like:
Fighters fire omega beams 1/round for 5d6 at 1st level