Flapper valve and soft rev limiter
Two things I don't like about the RC51 are the the flapper valve and soft rev limiter. Most riders can probably just barely feel they're there, some might not notice 'em unless you told 'em about 'em. Since I know they're there though it really bugs me.
I know the flapper valve is there for noise regulations, but sometimes you just have to let the v-twin roar even more.
I'm not sure about taking off the rev limiter though. It's at 9,500rpms (redline 10k) and I'm sure Honda has its reasons for putting it there.