The problem is not so easily simplified. It is a combination of a lot of bad things. For example i bet a lot of those "i left because of ganking" people would not leave, if either they had a chance to strike back or there would be more interaction between agressor and victim (more than "lol! gtfo l2p!") or there would be some reaction from the gameworld (not exploitable guards!) or fair statistics.
So unconsensual pvp alone is not the problem (for "some" it can even be motivating). Still it should not be something actively promoted and rewarded progressionwise (factionwise or renownwise is a different matter) by the gamesystem.