There are certain weapons and such that give critical damage whenever you meet a certain requirement (i.e. Pyro's backburner gives critical damage if you're hitting them from behind). 100% crit server means every bullet does critical damage. I'm not sure how much critical damage is, but its enough for a few classes to be completely overpowered (sniper, demoman, soldier) and making other classes completely useless (scout is all I can think of at the moment).
DFO: Astenhof 35 Soulbender, DoomPig 14 (going mechanic)