Trades don't start coming out until I believe late next month, early June. I'd check out the trades to Snyder's Batman, Morrison's Action Comics, Azzarello's Wonder Woman, Johns Aquaman (it actually is a good book), some of the other titles that others have praised in the thread like Flash, Batwoman, etc.
Would I consider Justice League a "gateway book"? No not at all. Nothing in the origin arc really ties in to the other books since Justice League takes place five years in the past of the New 52.