Prone: You can tumble on your feet, and you can tumble while prone, but you can't tumble to your feet from prone. Or rather, you can, but it's a DC35 and you still get an AoO for your trouble. tripping is way too easy, and there is no way to avoid the AoO for becoming not prone again.
Tumble: dwarves can tumble in any armor, by the rules.

wtf??
They can do that but I can't do a shoulder roll or a kip up and avoid the AoO?
Honestly though, do you really need to be doing somersaults and cartwheels past someone to keep them from smacking you as you go by? Football players run around, juking, keeping everyone off their asses, none of which looks like tumbling to me. Does look like the basis for a combat maneuver to me though.
-10 you're dead: this one just really sucks. makes me have a new character every 2 sessions. great for continuity IMO.
-1 to -9 you're bleeding out 1 hit point at a time: often times, if someone goes down, the other say "well, he's at -3, we got plenty of time to kill all the bad guys and come bind his wounds at -9." so I sit there for an hour going "ok, rolling for stabilization..crap, -5, next"
Feinting: so you successfully feint, and this guy loses his Dex bonus for your next attack, but if you provoke an AoO from him, he can still take it normally. You just faked him out, and put him where you wanted him, but he still sees an opening in your defense.