Really bad postflop play is super obvious when you see it. Chasing without odds and then checking when the draw gets there. Bluffing only at the wrong times: when your opponent is committed, when there's no chance you can have a big hand. Not recognizing the obvious slowplays. Making tiny bets and then instafolding to a raise. Playing too passively, only winning pots when you have the best hand. Betting tiny with monsters and giving odds to chasers, then paying off for a full stack when the draw hits. Thinking 2 pair is worth a stack on a 4-straight board. Etc. When I find a table with a couple of total morons like this, I make a point of raising a lot less preflop after they limp (which I hate doing) ... they're going to donk off huge chunks of money postflop and I want to see as many cheap flops with them as possible.

Good postflop play ... I'm not sure I know what really good play is these days.