For a long time, I've put more emphasis on postflop study than preflop. This was with the assumption that it's much easier to get to a decent preflop game than a postflop game, and that more of the money is won/lost postflop. I think that's still basically true, but I also think that preflop play may be relatively more important now than it has been.
Aside from the obvious, which is that preflop mistakes set up even bigger postlop bad situations, a lot more money exchanges hands preflop than in the past. There's more preflop 3-betting and 4-betting in general. I'm also significantly looser and more aggressive, which changes the way people play back. And overall, there are more regs in the games I play than in the past -- not sure whether that's true overall, or mainly because I've moved up in stakes.
Even though I've paid even more attention to seat selection now than I used to, I've gone from a 19/16 guy playing on a table with 2-3 fish and 1-2 regs to a 27/22 guy on a table with 2-3 regs and 1-2 fish. I'm raising more, and playing against guys who know they should 3-bet me fairly wide. I 3-bet 7% myself now, and didn't even pay much attention to it in the past. Not all that long ago, the closest I came to a 4-bet bluff was watching it in a video. Now, if I'm not careful, I would be 4-bet-bluffing every other orbit :P.
So, the point is that it certainly pays to pay more attention to preflop. The question is whether there's a ton more for me to learn there. I don't think I know all, just saying that I know I have more to learn postflop, and I'm guessing I always will. Whereas I think preflop will be more subtle tweaks than anything else for the most part.