Two main differences between versatility and resilence are that versatility works in PvE, and that versatility has a portion of it's stat budget spent on offensive gains as well as defensive gains. The second point, the stat value I think is important to stress here. Numbers arn't balanced yet, so I'll just look at it in a vaccum. If versatility is greater offensively than mastery then the free defesive benefit makes it the best stat. If versatility is greater than stamina for defence then it becomes the most powerful stat because it has a free offensive component. If blizzard wishes to balance vesatility in a way that doesn't just make it the alpha and omega of pvp they need to make it inefficient offensively and defensively and only compete when you are literally looking for a damage reduction and damage increase at the same time in a sustained environment.
If X quantity of versatility is giving more effective health than X quantity of stamina then if will have a negative effect on percentage heals. Most cases where you could chose to gem resil or stam the resil actually gave you more effective HP increase than the Stam, so the resil is just simply a more powerful stat. This is much less likly to happen with versatility as it also works in PvE at full power, if they overvalue the stat as resil has been in the past then every PvE tank will be going for as much versatility as possible and avoiding the other stats.
As it looks right now based on buffs about 3% versatility is worth the same as 5% of crit/haste. As long as stacking it has less offensive gains than a pure offensive stat and less defensive gains than a pure defensive stat I can see a place for it in pvp.