One thing, (while maybe not a single rule), that annoys me is the +/- modifier. For example, make a heal check to perform brain surgery in under 6 seconds (with advantage), roll d20+4. Make an attack roll, saving throw, skill check, etc with a single or series of modifier(s), spells that give a +/- modifier, magical items, etc; see where I'm going with this? I really wish that D&D would have gotten rid of this by now and just made rolling a dice pool rather than having to add or subtract modifiers. It can slow the game down at times. 1d20+1d4, 1d20+2d6 would be so much easier IMO, or something similar.