<Snipped quote by Terminal>
Why do these bother you? And is this a three way tie? (If not, pick one.)
Tricky one there. All three make me feel different mixtures of rage, disappointment, and disgust. I suppose the biggest MAGNITUDE of all of those would count for the purposes of your question.
Which would mean my biggest pet peeve in this specific context would be Casual Roleplayers, because that disappoints me far more than the other two do, in addition to how much the other two make me angry and disgusted.
One of the biggest flaws I perceive to exist within the framework of RPG is the way in which people use the three two different 'level' subforums. More specifically, that the posted guidelines for each subforum are at best misleading. Most of the people on this forum write as a hobby, even in advanced - which in my opinion is detrimental and contrary to the spirit of that subforum's purpose. I would like to think that the Advanced Subforum should be a place where one strives to only put forward their best effort and nothing less. People look at Advanced and think that the only qualifier for posting there should be technical ability and skill level for writing, but the fact of the matter is that a lot of Advanced Threads suffer high turnover more because of posters who treat roleplaying as a hobby, post twice, and then vanish into thin air never to be seen again.
In that framework, I would define Casual Roleplayers not only as posters who have not yet reached an advanced level of writing proficiency, but also those who lack the motivation and commitment to actually take the effort seriously. By that metric, a significant percentage of the Advanced Roleplay population - perhaps as much as 75% or more - would qualify as casual roleplayers rather than advanced.
That is my problem with casual roleplayers. Either they fail to perform at an advanced level, or else they behave below an advanced level to the detriment of people who actually take roleplaying and writing seriously, while publicly projecting the exact opposite. The disappointment I feel at how much wasted potential is present is overwhelming. Throw in some disgust comes from how their behavior in the advanced subforum damages the advanced community as a whole.
So in a way, my problem is not even with people who actually frequent the Casual Subforum. If they think they are not quite up to advanced standards or else only go there because of concerns over time and commitment, that's absolutely fine. That's exactly the reason the Casual Subforum exists, and there is no shame in pursuing a hobby and building skill over time in a (somewhat) consequence free environment. What I dislike are the Casual Roleplayers masquerading as Advanced Roleplayers who refuse to take the concept seriously. As I said earlier - if they are not prepared to put forward their best effort, then they should not be in the Advanced Subforum! We have another Subforum for that! It's called Casual!
Part of the problem is due, in my opinion, to the listed guidelines for each of the subforums. My entire rant here is based wholly on an opinionated viewpoint of what the guidelines should be, not what they actually are - and so my problem is baseless in the sense that the people I accuse of being Casual roleplayers are, by the definition of the suggested guidelines, actually Advanced. The reason I see this as a problem at all in the first place though, is because the issues I describe above are (again, in my opinion) caused in part by the inadequate guidelines.