Some have been said but, in no particular order...
RC Marketing (ok, this is probably one of my top 3). Some of the crap they say is just SO stupid and I feel sorry for all the noobs that fall for it. I see how in a way, that's just how it is, but I sometimes get tired of reading "the best and most powerful ever" or "strongest ever" on every parts description I read.
Stripped screw heads/threads. I f*cking hate dealing with this. Can't they make the thread coarse enough when it's going into plastic and put a large enough head on the screws? Pain in the ass dealing with 1.5mm hexes, as if 2mm wasn't small enough anyway. (Alright, another of my top 3)
Noobs on forums asking questions just to "look cool." It's so damn obvious, and so annoying.
Noobs who don't listen to you despite the fact you are obviously more experienced.
Noobs on forums thinking that having 17,000 posts of "I don't know" in various threads makes them an RC Expert.
People who want a 100mph RC yet expect to do this with 50 bucks. And when you find out they are racing on a track and you tell them "there's no point" they say "I can handle the power."
People who "know" their RC goes X fast despite having NEVER actually used a GPS or a radar gun. "Yeah, my 100% stock 2.5 T-Maxx does 60+mph." Ok. If you say so buddy.
The same...questions...over...and over...again. Be it on the forum or when someone asks you while you are driving. At least we don't have much of the repetitive noob questions on RCM, they all seem to be stuck on the Traxxas forum.
LHS (in a way) - A part that's $1.50 off tower is $6 from them. I mean c'mon really? I know there's a convenience charge but damn. On the plus side a fellow RCM member works at my LHS, makes it a lot better.
The stuck up nitro guys. Some nitro guys are fine, and admit that the proper brushless systems will obliterate their vehicle, but still prefer the sound of nitro and the fact that they have an actual piston thumping in their RC. I totally respect that. I in a way have the same feeling and own some nitro's (haven't been driven in forever though). But I hate when you get some fool who thinks electrics are for kids and shouldn't be allowed on the track. Well, Mr. Fool, I just beat you down the straight by a mile, and I just jumped OVER you when I turned the entire rhythm section in a single jump. Like to rethink your statement?
When you are out driving and the guy asks you the usual questions and when he finds out how fast it is and how expensive it is, he says "yeah well my Mustang is this fast."

(Alright, this isn't something I don't like, cause it's so damn funny when it happens).
There's no perfect connector. It either doesn't support current worth a damn (Tamiya), is a pain in the ass to solder (Deans and Traxxas), or is too big (Powerpoles and Traxxas).
Servo wires. Either WAYYYYYYYY to damn long or 1 inch too short, but always the right length to cause as much trouble as possible.