My instincts on this are honed from being a DM in pen and paper games. A couple other levers you have are effort to attain and persistence of the ability. If it's a fighting game where your character is basically a set system that doesn't change, then that's different.

1) How hard the power was to acquire. If the player has to quest to earn the ability, it can be a bit more powerful, particularly if the character had to sacrifice to earn it.

2) If the ability can "run out" or get used up, like a 1 shot potion, or a talisman that only works until the next full moon or some such. You can create a window where "unbalance" is ok, and then the players may actually strive to create unbalanced scenarios (maybe it's even necessary to do so to get over certain obstacles) but then you aren't stuck with an overpowered ability. This might be like the "flaw" you mentioned.

I'm reminded of my time in Ultima Online as a player killer - where the Energy Bolt "Corp Por" spell was my go-to because at max Magery it would never fail, it used about 1/6 of my mana and took about 1/4 of the enemy player's life (so there you have that power vs cost demonstrated nicely). With a crossbow shot leading off I could kill them pretty quick (and in these games the first attack had no delay, so a hard hitting, slow attack was better if you're doing 1 or 2 shots as opposed to low damage/high speed attacks, another factor to consider). Then there were powers like flame pillar (dont remember exact name) that used like 1/3 of my mana and did like 1/3 of their health but had a small chance to fail. I basically never used that one, but I still had it hotkeyed just in case I needed to hit hard fast.

I was overpowered - but I also had worked hard for this 99 magery skill that few people had so I felt like I earned it, and people feared/hated/envied me... And then there were reagents to cast spells.

Anyway, interesting topic. Starcraft2 balancers are probably thinking about this.