To carry Burniel's thought on fastpass systems further;
What grinds my gears are poorly planned fastpass systems, such as Cedar Fair's current program.
While Six Flags, Disney, etc. use a fastpass system that considers queue length and gives a time estimation to return back to the ride, Cedar Fair simply has another queue built for FastPass-ers to wait in, which theoretically shortens their wait. However, since Cedar Fair parks are not moderating which rides FastPass-ers can queue for, this inevitably means FastPass-ers wait in longer FastPass lines for popular rides (as they cluster their, same as the rest of the park guests), while less popular rides remain walk-on for FastPass. This causes inefficiency in the system; the more equitable system would employ a moderation of the popular rides, allowing FastPass-ers to queue for those rides while also taking advantage of short queues for less popular rides throughout the park. For instance, the Six Flags Flashpass allows guests to virtually queue for popular rides while waiting in normal lines for others. Cedar Fair, instead, only uses a price premium for access to the most popular rides; this however does not truly moderate queues. And since Cedar Fair has FastPass-ers still in the queue, this requires far more complex logistics for waiting than a simple pager system that allows guests to simply slip in when their time to ride comes.
I also hate assigned seating.
It can be simple enough to have a ride op man the queue line, to limit number of riders coming into the station. But assigning seating, especially ride ops who are very adamant on the assignments, always takes away from the experience.
Finally, I hate rides that do not have an MCBR.
Yes, hypocritical as I have a number of highly ranked coasters without MCBRs. However, there is so much value to coasters that allow for multiple trains on the course. Even shaving 10-20 seconds off on dispatch times from MCBRs counts, especially if you are waiting for 20+ cycles before riding.