Originally Posted by
Jester Joe
You can use Old Gods in Wild.
You can't use all Wild cards in Standard (Ex- GvG/Naxx). You don't have to have Old God cards to play Standard, since it still has the pool of TGT, Blackrock Mountain, and League of Explorers which you can make solid decks with.
Basically, Standard is to keep a limited pool of cards and allow greater balance (And arguably accessibility, since it will save newer players from needing all the GvG/Naxx cards now), while Wild is so you can go completely crazy with all the cards.