The new template system allows Blizzard to quickly turn the dials on and tweak specs quickly--and without the risk of impacting their PVE balance. While I think in the long run this is a good thing, it also means that changes will probably be faster coming than we've ever seen in the history of the game. Just over the past few weeks, we have seen how quickly shipping out a few small adjustments can drastically affect the meta. DKs, Mages, DHs, and Warlocks have all been impacted.
As we enter the final day pre-launch, many of us are still trying to settle on a main. My advice is to not pick your class based on how it performs now or in beta, because if it is OVER or UNDER-powered, chances are pretty good Blizzard will be making changes--now more than ever, because it has become so much more easy for them to.