A few things really.
1. Text speak. I can barely tolerate it in actual texts. Anywhere else, and I just can't deal with it. I don't care if you actually said something intelligent, text speak makes you look(to me at least) like a teenage girl who couldn't pass their third grade english test.
2. Undocumented code. As a kind of new coder, it annoys me to no end to see code that has no documentation or comments. If you know what your code does, there is no reason you can't put a couple words next to it in a comment that explains what it does...
3. Kinda goes along with number one, but bad grammar and/or spelling. No, I'm not talking about grammar nazi levels. But if you are posting on a public forum for thousands of people to see(Not just Iwaku, but sites like reddit and even Facebook), the very least you could do is follow your browser's built in spell check.
4. Cursing in a argument/debate. Soon as the first f bomb is dropped, as far I'm concerned, you've lost.