Usually if you're a long time member and the infraction is minor, such as language, trolling, flaming, or cumulative actions of a nuisance nature, you're going to get a warning. You MAY but may not also get warnings for things like misleading posts or things that imply ways to get around forum rules.
Bans immediately come for things like porn, spam, advertising, corporate selling, repeated violations of trolling, flaming, drinking caustic substances, telling someone to go die, cumulative nuissance issues after being warned in the past, being a general pain to the staff. Generally we warn you once, and that's it. After that, sayonara. More latitude is generally given to people who have been here a while and contribute useful posts regularly. We do realize that people get into heated debates or arguments or sometimes skirt a gray issue area. Warnings and Bans are in a gray area and what mods do in some forums may not apply in others due to the propensity for certain hot button issues or problems to escalate in one forum more than another.
For example, if someone comes into the Bob Long forum, that I moderate, and makes a thread saying that Timmy's suck, I'd probably close the thread and give them a warning. The posts in there tend to be pretty well taken and the members in there are rather sedate and patient. If a similar post went into General Paintball Talk or High End Guns and the user had like 2 posts, it wouldn't be unexpected or unreasonable for CoolT, UTLadiesMan or another mod to close the thread and ban due to the fact that it's likely the person is deliberately trolling.
A lot of the ban vs warning thing comes down to intent. Are we going to get it right every time...no probably not, but I think we do 99% of the time. The staff has better things to do than police posts that have issues with them. If it's something you wouldn't say in front of your parents, small children and your priest, then it's something you should think about rephrasing or not posting.
Bottom line is that if you see a post that is out of line, click the "Report" button so that it's brought to the attention of the moderators. Making posts in a forum or responding to trolling or flames only makes more work for us. We do proactively check the reported posts log.