If you don't introduce yourself.
If you jump straight in to a controversial thread and I have no idea who you are or what you are doing on a particular forum, without having introduced yourself first, I will ignore you. It's like a stranger coming up to you on the street and yelling in your ear. Rude.
If you don't write in paragraphs.
You may be the nicest, most polite and most deserving person on the forum. However, if your post is one long page of unbroken text, I won't read it. My eyes don't like it and my brain won't process it. Write your heart out, sure. Then before you post, go back and separate your post into paragraphs. Leave a space between paragraphs. Then I'll read it and probably respond.
If you are ungrateful.
I may have taken the time to respond to your query, comment, argument or concern on a previous occasion. If you responded with " Wow. You totally missed the point. I don't know why you even bothered to respond, you arrogant woman." or similar, I will ignore you from then on. If you think a response you've been given is stupid, keep it to yourself. Unless it's hurtful or abusive, in which case, go for it. Remember that a stranger went out of their way to consider your problem and took the time to try to be helpful. You don't have to lie. You don't have to gush. You don't even have to respond. You just have to move on. Quietly.
If you expect me to understand everything about your situation from one post.
If there are details that provide important context to your post, include them. I don't have access to the inside of your head. I can only respond based on the information you have given. If you realise you forgot to provide the important details, graciously explain that in a subsequent post instead of getting huffy with me because I didn't somehow guess them. If you do get huffy about my lack of psychic abilities, I will ignore you.
These reasons are brought to you in a spirit of slightly annoyed helpfulness.