Why PvE/PvP mix?
You could have answered this question yourself. It's all about the stats you are getting : more stats = more pew pew, right?
The PvE gear will usually offer more sockets to put gems in. Eg the legs from SWP gives 3 sockets and great stats which are way way better than the PvP ones.
So you just compare the stats, if you a PvE item gives more, you pick over a PvP item and vice versa.
The reason why we are not fully PvE geared is because the PvP gear gives more stamina and since people can crit for 10k+ you need stamina at the cost of better dps in order to survive. (except for some comps where the dps will use full pve gear)
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As for the most played specs you are right. This bracket is filled with paladin prots, rogues and disc.
Other most common specs are: Boomies, Hunters, Ele shammies.
Classes avoided usually are: Warlocks and monks
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The thing is that almost all specs and classes are viable at 70 if played right. There are 2.2k warlocks and ferals druids, but kids will just roll the most OP comps which ends in destroying the bracket a bit more everyday
Idk if that helps. Good luck!