1. Advertising - basically anything that doesn't in add to the site, like "go to my forums" or "support my site"
2. Off-topic - going off-topic in a thread is considered a form of spam, so remember this when you feel like going off on a tangent.
3. Posting about a previous subject - starting a thread that has the same subject as others in the same forum is spam. Use the search bar.
4. Internet Slang - Excessive slang or leet speak is considered spam, since it can cause problems with effective communicating.
5. Double-Posting - Posting twice in a row is considered spam. Use the little edit button. Yes, the magical little button at the bottom of the post.
6. Unneeded Comments - "haha", "LOL", "*giggles*", pretty much anything that doesn't contribute to the thread subject.
Also one other thing that has been getting on my nerves is the subjects. "Check this out" is not a subject. People need to use the subject line to discribe what the thread is. It is not spam but it is something that is being one alot.