I discuss my top 5 design flaws for PVP combat of MMORPGs.
In reverse order, they are...drum roll please...
#5: 50% healing debuffs
E.g. Mortal Strike for pre-Cataclysm Arms Warrior in WoW. When 50% healing debuffs are part of a game, healing is stupidly easy when opponents don't have the debuff, but healing is stupidly hard (and inefficient) when opponents do have the debuff. As Blizzard figured out by Cataclysm, the better solution is to reduce the effectiveness of healing debuffs and healing and mana regen together, as this leads to improved balance and better gameplay.
#4: Asymmetrical damage scaling
E.g. Bright Wizard's Combustion mechanic in WAR. Combustion caused a BW's burst and sustained damage to ramp up significantly and scale with gear, whereas the downside (explosion backlash damage) was fixed. Therefore, the benefit greatly outweighed the downside.
#3: Asymmetrical CC
Relative to your opponent, this is having too much chained CC capability (e.g. stunlocking, chain-fearing) or having too much CC immunity (e.g. RIFT Pyro Mage's Ground of Strength indefinite CC immunity).
#2: Overpowered passive procs
E.g. Lingering Wounds, Vampiric Munitions, and Fell Blades from RIFT. Strong debuffs or effects should be tied to an ability with a meaningfully long cooldown, or there is no thought needed on the part of the player, and the result is skill-less PVP.
#1: World PVP based on 2 factions
This is too fragile when there are population imbalances, which tend to further increase the imbalance over time because the winning side gains PVP rewards faster than the losing side. See SWTOR on any PVP server.
I'm sure that I missed other design flaws (e.g. gear dictating outcomes, etc) that chap your hide, so let me know which ones I've missed and what you think of my list above.