X-Men movieverse is my fandom, but some of these apply to slash in general.
1. Male superheroes who sound like girls. And not even real girls, but stereotypical girls, at that. These people are in a paramilitary organization. They're tough. Having sex with a man, or being a bottom in sex, does not translate to feminized behavior. It just kills the willing suspension of disbelief.
2. Homosex that sounds like heterosex, only with two guys. Anal sex really isn't just like vaginal sex, and it certainly isn't just like vaginal sex as presented by a romance novel. Learn something about male physiology before you write m/m sex scenes. Which leads me to...
3. The idea that sex between men isn't sex unless it's anal penetration. Related to the above, a lot of heterosexual slash writers present blow jobs, hand jobs, intercrural coitus (go on, look it up) etc. as "foreplay" and anal sex as "the act" or "making love." It's an extension of the error of looking at m/m sex as akin to m/f sex, of course. In addition, I personally feel it's also completely discounting of women's experiences as much as men's to deem one form of this varied human endeavor we call sex "the real thing" and the rest just adjuncts. In any event, anal penetration is not the only sex act between men that counts. For some men it's a favorite act (on one end of it or both), some think it's okay, some don't like it at all.
4. Getting the New York details wrong. The X-Men are based in a NYC suburb. New York City is a big place - there are lots of people here and many of us would be glad to help people who want the details right. You want your X-Men to come to the city on the train, like they do in the movie? The train ends up in Grand Central, not Penn Station, not some generic train station. You want the kids to go shopping at the mall? Don't populate it with stores that aren't found in this part of the country. You want them in a restaurant? Don't give the restaurant a smoking section.
5. Distracting dialect-y speech. Rogue saying "Ah'm just so scared ah can't stand it!" Even when she's talking to herself, even when it's her thoughts not her words, even when she's writing in her diary. Nobody perceives herself as saying "Ah" when she pronounces "I". There are lots of Southern accents out there and you can convey that she has one by word choice and syntax, but when you start writing "ah" and "sugah" you make your character sound like someone out of a comic book!
Hey, that was fun! I try to keep the persnicketiness (or is it persnickitihood?) in check, usually, but it's great to let it out from time to time.