1. People who don't tell you about their drawing. They post something, and it looks good, but maybe it's a bit confusing and so you're left wondering about it. It would be nice to know things like who's in it, or what the characters are doing, or why one of them has a flamethrower. But instead of giving you context for their art to help the viewer understand it, the artist says nothing, as though everyone is just supposed to read their mind. This can really make or break a picture for me. If I don't understand it, I'm less likely to save and favorite it, even if it's good. On the other hand, I've favorited things I didn't initially like because the artist has taken the time to say something about it, giving me the necessary context to appreciate what they've done.
2. People who use their artwork's description as a personal blog. This is only going to work for the artist, and the people who follow them religiously. Everyone else, who happens upon their artwork at a later date, or out of sequence, is either going to be confused, or indifferent. Blog in a blog, where it belongs. This is especially annoying when the "blog" takes the place of real information about what they've drawn.
3. Deviant Art watermarks. Look, I understand that art theft is a real problem, and it's a good idea to sign your art in such a way that it's hard to get rid of your name, but there are better ways than sticking a giant logo on top of your picture. The DA watermark just tends to get in the way, detracting from the art it's trying to protect. Instead, I wish people would try to find a creative way to sign their pictures. I've seen people do it many different ways, often quite creatively, that don't obscure, or otherwise vandalize, their own artwork.
4. Giant resolutions. For some well drawn and highly detailed art, a huge resolution is a good thing because it lets you see all that detail. But if you threw something together in 10 minutes that looks like a 6-year-old drew something in crayon, we don't need a 12,000 x 9,000 pixel picture of it. Use a size more appropriate for the quality of your work.
5. Tiny resolutions. I've seen some great artwork that's simply too small. I go to click on it to get a bigger view, and there's nothing. It may have been uploaded at a large resolution, but the artist simply didn't provide a way to get a larger picture. These are often works that you can tell had a lot of effort put into them, but then the artist only posts a 600 x 800 size version of it, when you're sitting there wishing for something in wallpaper size. Some people do this to encourage contributions to their patreon, which is understandable, (if not annoying,) but other people just don't seem to realize that the 3k x 4k drawing they uploaded isn't showing up that large because they didn't enable it when it was published.
6. Galleries where everything is stuffed into folders. Folders can be a great way to organize your art if you do it right, but I wish people would also leave a copy of their art in the main gallery so you can just browse through it all chronologically. When everything is in a folder, and nothing is left in the main gallery, it sometimes makes things harder to find, especially if the folders aren't organized according to topic, or you have multiple layers of folders to sort through. Too many people divide their galleries up into, "Drawings", "Commissions", "2015", etc., and those names don't really tell you what's in them, making me more likely to simply skip them instead of wasting my time wading through them all. You might remember that commission you did years ago, but no one else will have a clue. Try and organize in a way that makes sense to someone who's not familiar with your art so they can find what they want, such as "My Little Pony," "Still Lifes," and so on.
7. Sketch lines. Seriously, if you're going to post your art, try to clean it up a bit first. Sometimes the sketchy look is ok, especially if you just want to post a quick sketch. But if you go to a lot of trouble to color and shade something, maybe you should put the same time into the lines as well. Particularly annoying are the people who do add clean lines, but then also leave the sketch lines showing, as though they worked on the drawing in a single layer. This tends to make otherwise great art look quick and dirty.