Well - the results are in and around 70% of the people (n = 120 as of me writing this) voted for the Turn as the most difficult street to play in NLHE.

As I was writing about postflop play, that was my intuitive guess and it's nice to have it confirmed. So I guess the question is ... why? Aren't river decisions the most expensive and thus the most critical?

I think the crux of it is lack of planning and thinking preflop and on the flop. Before I elaborate on that, let's breifly talk about each of the streets.

Preflop: Many players have static and speculative ranges when playing preflop. The most difficult situation people face when playing preflop is how to react to being three-bet. Other than that, most decisions tend to be automatic and inexpensive. It's also rare to make bet-sizing mistakes preflop (unless you are three-betting yourself).

Flop: Like preflop, many players tend to auto-pilot flop play. If they raised preflop, they often c-bet on the flop. If they called preflop, they will continue if they hit and give up if they missed. Bet sizing tends to also be fairly automatic. IMO, the flop is the most critical street in NLHE. Planning the rest of the hand needs to happen here.

Turn: The turn is the first time I think most players start thinking about ranges (something they should have been doing preflop and on the flop). Thus, the turn often comes to players as a shock if they were on auto-pilot for the first two streets. They don't plan their hands and now are forced to make decisions. Since they didn't spend too much time thinking about ranges preflop and on the flop, now they are forced to work overtime with the timer running. This creates a stressful environment and that tends to be memorable.

Also, the turn is the first time most players intuitively experience their preflop and flop leaks starting to compound. A lot of people think they have a pretty solid preflop game (99% don't) and are pretty good on the flop (where planning doesn't happen for many), so they attribute their discomfort with the turn specifically to the turn and not to the combination of the first three streets.

Finally, the turn is also this mixture of the pot already starting to get expensive and many unknowns still up in the air. There is still one more card to come, and many players tend to follow through on bluffs on turns.

River: The river is interesting because even though you will see it least often, the decisions you make there are often the most expensive. So why wasn't it picked as the most difficult street? For one, there are no more unknowns when it comes to the board and thus there isn't much to speculate over if you were drawing.

Player's ranges on the river tend to be fairly polarized - which is also true for many people's preflop and flop ranges. The turn, as "ienjoypoker" on 2+2 observed, tends to have the most situations in which players merge their ranges as they semi-bluff. Very few people turn their made hands into bluffs on the river or value-bet thinly at the micro and low stakes.

Finally, since most players started thinking about their opponents ranges and hand reading on the turn, they now can follow-through with that on the river. Thus, unlike the turn, the river is almost never the first street that players mentally engage on.

So that's some of my thinking on the topic ... let me know additional thoughts/points which I might have left out.