What are the limitations in combat?
You cannot transform into Superman and shoot lasers from your eyes et.c.
You cannot actually lift mountains, or destroy mountains, no matter how high your mining or str or combat skills are.. (to throw at an enemy)
No.. unsure what you really mean, but... There is a SDI cap in PvP, none in PvM.
There is a cap on armor ignoring damage you can do in PvP (might be in pvm aswell, but it depends on max hp, and monsters usually have ALOT, so its usually never visible)