I think 52 is the easiest point to jump on - they tried to make it that way. I think booster gold and green lantern have continuity references....booster gold jumps through time and includes such famous parts of DC such as Killing Joke and Zero Hour, but you should be ok.
JSA is probably easiest to jump on (the 1999 series that spins out of morrisons JLA crisis times 5 arc)..it's also co-written by David Goyer (Blade/batman begins screenplays)...the current action comics I read a couple of arcs...a bizarro thing which i didn't like, and something involving legion which was good, but kinda continued on from the Lightning Saga crossover in JLA/JSA.
Thing with Johns is apart from Infinite crisis and some others he's mostly on a book for a load of volumes, and I suppose part of the reason he's so good (and the problem for new readers) is that he makes references to all the continuity in DC but makes it all seem like it was planned rather than editorial decisions. This is why rebirth and booster gold are probably his best comics - he made hal good again, and he made booster gold cool. He's also got the take on justice society being more of a family, and you can train the young ones and there're certain titles that get passed on - made it a genuinely good book, rather than like a first attempt at a justice league. He also did a good job fixing power girl and the multiverse - so that pre-COIE stuff is still relevant.