Rationalists often presume that it is possible to do much better than average by applying a small amount of optimization power. This is true in many domains, but can get you in trouble in certain places (see: the valley of bad rationality).

Rationalists often fail to compartmentalize, even when it would be highly useful.

Rationalists are often overconfident (see: SSC calibration questions) but believe they are well calibrated (bias blind spot, also just knowing about a bias is not enough to unbias you)

Rationalists don't even lift bro.

Rationalists often fail to take marginal utility arguments to their logical conclusion, which is why they spend their time on things they are already good at rather than power leveling their lagging skills (see above). (Actually, I think we might be wired for this in order to seek comparative advantage in tribal roles.)

Rationalists often presume that others are being stupidly irrational when really the other people just have significantly different values and/or operate largely in domains where there aren't strong reinforcement mechanisms for systematic thought or are stuck in a local maximum in an area where crossing a chasm is very costly.