How the hell does DotA have such large prize pools when so few people play it by comparison? IT MAKES NO SENSE.

As for the difference from DotA to LoL, the impression I always got was that a game of League tends to move at a faster pace (can't deny creeps = more gold = full build faster), that League is more streamlined (less active items, a more definite meta, better balancing, essentially what Hank said - it makes more sense), and that because of that League is without doubt an easier game to play and master. To be honest I think that's what made it popular - it took what the original DotA did and made it more accessible, whereas DotA 2 didn't. 

My roommate plays DotA 2 and, naturally, detested League, but when I watched him play it just looked like so much freaking effort. I reckon it's possible to play a casual game of League. I don't think you can play a casual game of DotA, judging by my roommate and the way he spoke about it.