Top 6 things I find annoying/dislike about Internet forums (because I couldn't think of 4 more to make it 10).
1. Excessively large/long signatures.
The most annoying thing about forums is when a short one-liner post is trailed by signature containing a large banner image. Even worse would be if the person is a frequent poster and has multiple large banner images in their signature in bitmap format. I read forums for opinions and information, not to see the same large image repeated multiple times. This is why I always disable signatures in forums where the option is available.
2. Using a text colour that is difficult/annoying to read.
In most cases, people who are stupid enough to post in bright yellow font on a white backround usually don't post anything worth reading, but it's still a pain on the eyes.
3. Large images in posts.
Not only do large images tend to break forum layout, causing one to have to scroll horizontally to read other people's posts, it's also annoying for people who have a slow connection. When posting large images, post it as a link or use a thumbnail.
4. Unnecessarily long quote ladders.
A quote in a quote in a quote in a quote in a quote in a quote. It's very annoying to see a post that is reponding to a single person, but they include all the quotes that the person has included in their post. It should never be necessary to include more than 2 quotes in a quote ladder. And if it's a long post that you are responding to, only quote the parts that you are addressing.
5. People who don't read other replies before posting their own reply.
"I didn't read the thread, but I think…" Usually people who do this don't care what others have to say, and often they end up posting what has already been said. I make an effort to read the entire thread, and if it's a long thread with many replies I read at least the first few pages and the last few pages before posting.
6. Off-topic posts and spam.
Complete waste of time and space when people post something in a thread that is unrelated to the topic. Similarly, "I agree" one-liner posts (usually accompanied by annoyingly large signature) are annoying.