As a Musician in General
1.) People whom are living legends in their own minds. Been in bands with these sorts of people - most of the time they are the singer/primary songwriter and think they write "hits" and that we are going to become the next top 40 overnight sensation, when in reality, and in a lot of cases, just another guy who wants to get rich/famous quick. Before too long with these people, it becomes more and more about the sex/fame/money than about the joy of making music and the "magic" gets sucked out of the project by that attitude. A lot of these types have no idea how the modern music industry works either.
2.) People who refuse theory because it's not "punk rawk" or some other crap like that. It sucks when I try to explain something technical musically to someone and they can't understand it because they either developed their own language, or just play random stuff but can't explain how it works.
3.) People who are snobbish because they know theory. Theory is for inter-musician communication and nothing else, it does not make you a better person for knowing it, it just makes you a better musician to convey to other classically trained musicians. I kind of ride this ridge somewhere in the middle, and just keep learning as I go. I know how to read staff and tab and 99.9% of the time I still do everything from memory because it's faster. Does not make me lesser, just makes me efficient.
4.) Musicians who bring their significant others into band decisions when the S.O. is not a musician or not involved in the project. All that is is a catalyst to drama and causes all kinds of annoyances.
5.) People whom seem to think that controlled substances are an important part of making music. I don't like babysitting a stoned out drunkard who can't play. Have had to do it too many times. You know what sad is? A 28 year old babysitting a 38 year old alcoholic bass player whom is probably on way more than that.
6.) This fucked up contradictory view on money vs. wholesome reason to be performing. Ie. the catch 22 that if you make money as a musician that's what you are after, but if you don't make money you're a no talent hack - who are you to judge based on what is coming in and going out. Even Sonic Youth and the Sex Pistols played for money! Even hitmakers like Journey and Lady Gaga do this because they enjoy what they are doing.
I think a lot of musicians need to stop being judgemental pricks. It'd make the world a better place.
As a Guitarist
1.) Gear Snobs, my #1 enemy. They always pester me to buy this or play that because it's "better" than what I have. Um, I've worked DAMN HARD to figure out what gear I like to play, go buzz off and go shill your Les Pauls and Marshalls elsewhere bub. If anything, I love to tell these guys that it's not 1979 anymore, amps have more gain, you don't need Humbuckers to shred anymore, and Kemper is going to make their oh so special "Valves" obsolete. Don't like that I play a Chinese amp I modded myself, or wacky angular "gumbyesque" "Kirt Kobain" guitars - well, nobody is telling you to watch me, why not go visist some friends with a beer at the bar instead.
2.) Non-Musicians commenting on how many guitars I own. I own 30 because I BUILD, PLAY, and MODIFY existing guitars. I also sometimes sell my creations and want to make some hard to find models accessable to a handful of people who like them if I feel like. Yes, I'm aware I'm not Paul Reed Smith, Tom Anderson, Les Paul, Leo Fender, or Ned Steinberger - and I don't care or really want to be. Most of these people are the type of snooty Bellevue/Park Avenue/Hollywood types whom drive a BMW and have to live a "status symbol" life so if it's not 100% about money, it's not worth "investing" time into. Have you ever heard of "hobbies"? Maybe you should get some other than judging people on what they do with their free time, stop looking at everyone else and start taking a look in the mirror. Guitars are a passion, not a profession for me currently.
Crossing into non-musicians actually because of the above...
3.) I'm over 30, so a lot of people think I just need to cut my hair, hang it up, make a few babies, and "Grow Up". Well, that might be YOUR dream, but it's not mine. I'll keep doing this till I die. I'm not after money (already have it, I do I.T. for a living), I'm not after sex (I have a smoking hot wife), I'm not so crazy about fame, what I'm after is doing what I enjoy doing, creating/playing/performing. Sure, I'm not selling out Madison Square Garden, yes I'm just a little peon playing guitar on the small time local dive bar scene, but I ENJOY IT. That's why I do it. If I ever got bored or tired of it, then I'd quit and go do whatever else I'm into.
4.) Just because I'm into doing musical things does not mean I don't need to take a break every once in awhile and pursue other things. A lot of people know me as "the man of 1000 hobbies" but really that's just how I nurture my interests all around. Sometimes I need to put the guitar down and pick up a keyboard/mouse and study for I.T. work and my computer passions instead, or video games, or handyman things like fixing things around the house/car/whatever. I'd say I do a pretty good job at staying well rounded. Yes it generates a lot of clutter in my life, but that clutter is only temporary.
So there's some pet peeves I have to deal with as a musician....true love is having such a long post like above and being willing to keep going despite.