Trims that hit hard enough to noticeably slow the train down. Can be annoying with B&M hypers but the one that sticks out in my mind right now is The Beast. I was riding it a few weeks ago and all of the trims hit extremely hard and ruined the drops, especially the drop into the helix.
Uncomfortable restraints are also another big buzz kill. I was at Hersheypark for the first time recently, and I felt no need to ride Skyrush more than once because of the restraints. Shame because the layout itself is fun, but having your legs crushed more and more with each instance of airtime is not. This applies to the RMC single rail restraints, my legs got absolutely crushed on Jersey Devil and it’s my lowest ranked RMC as a result. Same for the Mack auto lowering restraints. I’m not as crazy about Time Traveler as some people seem to be, and part of it is from those restraints again continuing to tighten to an uncomfortable degree.