Whoa, lots of peeves on this topic x3 As for myself I have a few that I do not particularly like. So, lets see how long my list of peeves turns into...
1. I do not like poorly constructed costumes. IE half assed ones. For me at least, I take pride in making my costume, and trying to be as accurate as I can be to the character I am going to portray. I stop at no lengths to try and make it as perfect as I can, and even then I usually have things I can improve on for the next time I wear a costume. Wigs are a big thing, and since I normally have butt length brown straight hair, I have to buy wigs for my characters. There was only one character I have worn that had that long of hair, but since my hair was not enough for it, I had to have a wig anyways to help make my hair look fuller for the cosplay. (charlotte) I find it annoying that some people have cosplays without wigs that match the character, and either do not put their hair up at all or do, but it is the wrong color/length/look etc. I can understand that funds would be an issue, or the lack of being able to style a wig that would be either very complicated for a beginner wig styler or waaaay to expensive for a professional to make, but it only goes so far... Even if you cannot make the costume, at least buy one that resembles it more. It may be cheaper, but cheaper does NOT always mean better. The picture could be inaccurate, etc. So one thing is do a lot of research into your cosplay, and find people that do make it that make it WELL. Sure it might cost more, but it might avoid some unwanted issues, IE people making fun of your cosplay.
2. The creepers, stalkers, clingers, and etc. I do not like random people hugging on me, touching me, or standing to close to me when they KNOW what they are doing WHEN THEY DO NOT ASK POLITELY (about the first two). I love giving hugs out to people who are kind and ask politely, because I am just a lovable person like that, but its when they wont let go, or are grabbing me without asking that makes me want to bite their head off. x__x If I know you for a long time, then its not so bad, but when some guy way older than me wants a hug and looks like he would be a creeper about it, then its not fun for me and makes me very uncomfortable. This also goes with people trying to date at a convention. Yes, we have common interests, but that does NOT mean I am looking for a potential relationship with someone I have not ever MET before. It happened to my friend one time, during K-con's 2007 Masquerade Ball. This guy was talking all nice to my friend, flirting nicely, until he insinuated that he wanted to go back to our hotel room. NOT COOL. We did not go to a Masqerade Ball for a one night stand D<. That was one of the worse things that happened. If we say no to a hug, or any kind of touching we feel uncomfortable with, that means NO. Period.
3. Completely rude/elitist cosplayers. It has been said before in this forum about them, but I figured I wanted to throw in my two cents with this as well. Generally, we understand that there are some cosplayers out there who are unbelievable good (nikkiolie >w< ), and while a good portion of them are kind and nice when you talk to them, there are some that are just down right snobs. I fortunately have never had the chance of running into them, but some of my friends who cosplay have told me about them and it irks me terribly. Just because you look amazing in your cosplay, whether you made it or not, does NOT give you the right to be rude to others. Its that kind of selfish, rude behavior that gives some con goers the nerves to speaking to cosplayers who they think are amazing. It gives you the thoughts like "Are they going to be rude to me?" or "I really want their picture, but I am afraid that they will be mean to me for it." or even "We are cosplaying the same character, but hers/his looks so good. Will they nit pick mine?" etc etc. Some people pour their heart and soul into a cosplay, and even if it isn't perfect, they love it and should be given the credit for it. Just because you (elitist mean cosplayer) bought yours and look amazing does not mean you can rub it into others faces.
4. Convention space. It may look huge with no one in the hallways, but it gets JAM PACKED during convention hours. I explicitly hate it when people stand in groups in narrow hallways and do not let others pass. Its even against the convention rules, but people do it anyways. People crowded into such small spaces makes for others stepping on cosplays, breaking props, ruining pieces etc. I have had my dresses stepped on countless times, and even with my dress getting dirty afterwards, there are some people who do not even say they are sorry! People should be considerate when seeing someone with a rather large dress, etc. in the hallways and should be wary of stepping on costumes. They rip, they get dirty, and it even leads to crying at some times. I know my Serenity dress has large wings on her back, so I am going to be careful this year and stay out of crowded hallways, but even then people bump into them and bend them without saying excuse me or sorry. It happened at Anime Expo this year, and it made me thoroughly peeved. Its called being polite with manners. -_-;
5. Last but not least, people who do not be quiet during panels, contest, etc. I want to hear what they are saying, not what you are saying. I have sat in audiences where people were constantly chatting, or yelling and screaming when their favorite character comes on stage. Its nice to let everyone else know you love them, but when they are speaking, please be quiet. I want to hear the skirt, I want to laugh too, and its no fun if I don't understand what is going on. Its common courtesy to keep your mouth shut when someone is speaking on the stage to you. Be nice and listen, clap or cheer when something good happens, but do it at a reasonable level for everyone else's sake. ._. Also, I do not like it when people stand up in front of me when I am sitting down. Yes, they have a good view, but then mine is blocked and I have to be the meanie to tell the person to sit down so the rest of us can see. Then arguments ensue, and its just no fair.
These are just a few of my pet peeves, but overall I love conventions and enjoy going to each one I attend.
I swear I am not a meanie either ._. I just have been brought up a certain way that I feel wronged when people do not respect others...
Well, my rant seems to be very long o_o Sorry about that everyone ^^;