I did extensive analysis on this but pretty much no one seemed to care about this ;-) Read here:
http://www.diablofans.com/forums/diablo-iii-general-forums/diablo-iii-general-discussion/208767-patch-2-4-2-gr-keys-and-other-changes-what
About the "which build are we running on T13" - it depends less on your build and skills if you can run T13 but more on the power of your gear, gems, and paragon.
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http://www.diablofans.com/blog/184/entry-512-arcane-buff-wizard/
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Monk
Demon Hunter
Barbarian
Wizard
Witch Doctor
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Spirit (Monk)
Fury (Barbarian)
Arcane Power (Wizard)
Mana (Witch Doctor)
Hatred & Discipline (Demon Hunter)
Compare to D1/D2 orbs:
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You can customize your character with purchased gear.
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http://www.youtube.com/watch?v=8c2r3ZNNmO8
(courtesy of Don_guillotine)