As someone who plays a Fighter, I think the way critical attacks work are awful for game balance and will lead to situations where Fighters are very powerful for short periods of time and then useless for huge periods of time. This playstyle sounds miserable for other players and even worse for the Fighter themselves. As Tevas pointed out the huge damage spike seems totally contrary to the design goals of the system. It also is more likely to benefit NPCs which can spend all their abilities in a single battle.