So, since I've done a lot of traveling over the years it's hard to find one group to play RPG's with for a long time. Most of the time, I find myself roleplaying in an MMORPG for some tavern talk or using various chat/forum roleplay. That's kind of why I'm putting it here, though this can usually apply to tabletop roleplaying. That being said, I think if you're going to state something you dislike or take issue with- then you should at least be more than willing to provide a 'suggestion' or something. Also, this is an opportunity to perhaps speak on some of your own quirks.
I get irked when people play modern military characters like brainwashed mo-tards (that means 'so motivated they're retarded'). Not only that, but the people who do tend to play characters whose entire military promotion system is based on how many Cobras you kill or something. I don't really get angry- I'm not expecting them to be experts or super-realistic about it- but I can't help but try to tell these people military guys and gals are like any other guys and gals- just a bit louder and with awful haircuts. A lot of people practically roleplay a 'bully' or a guy that just barks orders all the time and has no actual leadership skills. Believe it or not, if you're a dick to everyone you don't get promoted. Sure, there are some jerkoffs with bad attitudes in the ranks... but rest assured, if you're just a complete self-centered asshole you're a prime candidate for a uh... 'broadening assignment', which sometimes means "we're putting you away from actual units that do things so you can't rack up as many complaints from the people that have to work with you". So my guidance to people who want to play this sort of role is... well, be 'a person'. Most level-headed and sane guys in the service after a few years usually just wants to be treated like everyone else and enjoy what I like to call the luxury of normalcy. Marines, Sailors, Soldiers, and Airmen all do normal guy stuff. They watch movies, play D&D (that was pretty much all I did in OIF 1 until we pushed), play video games, read comic books... or whatever it was that holds their interest.
Tavern/Bar plots. A mob of people sit down and start discussing battle plans. In front of other people. That they don't know. Sadly, even politely mentioning it to someone has gotten me rude responses... so now, when I do social roleplaying I just... brush it off. It also happens a bit in D&D groups- people go into a tavern and start talking about their quests... I mean, doesn't anyone in D&D go have a drink to relax (Yeah, I know it's not fun Roleplaying). FUN FACT: In one game I mentioned this in-character politely. The players' response was to spam me with PvP challenges and claim it was 'PvP RP' to try and kill my character. In broad daylight. In a public area. Yeah. When I do say something, I try to hint that I'm not trying to 'mess up' their roleplay or cause a problem- but a good GM or enterprising player could leverage that opportunity as an invitation to sabotage in the game setting. Who knows, maybe someone will say 'that would be awesome'.
I can't comfortably play with someone who's roleplaying a child character. It's nothing against people who do that. I'm not saying that it shouldn't be done. I'm not saying it's dumb. I'm also not saying that it's some weird pedo-thing. There's just something in my mind about an adult pretending to be a kid that makes me uncomfortable. I don't be rude to these people, I don't run away from them, I don't ignore them. I just... politely find somewhere else to be. At the tabletop I have never had this issue, but I kinda think it would be the same. And by 'child' I don't mean 'minor by modern standards but expected to function as an adult in the setting', I mean like someone playing a little kid.
One of my biggest discomforts was roleplaying in City of Heroes/City of Villains... and people would play as outright knock-offs of established characters. I don't mean some guy thought Batman looked cool so he made a Batman-looking avatar and just played the game... I mean people actually roleplaying as 'Totally Not Batman But Named After A Flying Animal And Is an Orphan Millionaire With Boomerang Weapons And Also Coincidentally A Detective Hero Man'. Some people would mishmash two characters together and try and pass that off as an 'original' idea. At the same time... I get it, people want to make a nod at a hero or two that they like. And it's really hard to say something to someone about this, because it's their character... even if it isn't very creative by my own standards, y'know? Usually, I've only talked with people like this when they're wondering why people are brushing them off and dismissing them... and sometimes I find out it's a kid or something, and I have to take a brotherly approach to it... maybe ask "What is it you like about Batman?" and try to help them refine their idea if they ask for help. I call this a 'discomfort' because it was really me just kind of assuming this was probably a kid or a new person, and I was watching people be a dick to them. I don't really 'hate' this... it was just always hard for me to sit through, because... sometimes it's a kid, sometimes it's a guy that'll go off about how it's pure coincidence and he thinks everyone else is stupid to think the aforementioned 'Winged-Animal Superhero Detective Orphan' is anything like this 'Batman' you guys keep talking about. (You think I'm exaggerating that, but it's happened with heroes that are just about as well-known).