Nethaera from Blizzard has written a comprehensive article on her Blog regarding Proving Grounds. I for one will be utilizing this feature once 5.4 goes live. Often you will find me testing out rotations at a training dummy, but if you aren’t a DPS spec then you are limited for places to “Practice” especially if you are a healing class and to a lesser extent a Tank. You can’t exactly taunt the training dummy to attack you.
The other cool feature is the availability of four different modes namely: Bronze, Silver, Gold and Endless
I for one think that proving grounds, along with the dropping of arena teams is almost a quality of life change in WOW. This along with Blizzard allowing cross realm teams to compete in arenas.