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"Jeff: Sure thing. The raid aspect of it is definitely a progression from Black Temple. We’re tuning it for people in Tier 6 gear, however there’s not an attunement on the front door. The first three bosses are accessible by everybody however they will be tuned for Tier 6, but there’s a lot more to Sunwell Plateau than just a raid dungeon too."
So Sunwell will be available to everyone (though expect much wiping without T6

). I remember reading that there will be a world event that will unlock the other bosses in the instance (who won't be available until then) too, somwewhere but can't remember where.