K, I think you summed it up pretty well. There are no hard and fast rules, however, these are the guidelines I use.
1. Does it adhere to the rules here? (I'd guess 98% of posts from other sites are removed for failing that first criteria.)
2. Does the User have to leave the site to read, understand and discuss the content of the message? I.e. in your example of "Ha ha look at this crap" with a link..." If yes, then I remove it.
3. Is it habitual? Many cross posts from another site? Then I usually shoot the User a pm first telling them to please stop.
4. Does the site the content is being quoted from have message boards or is it considered a competitor of ours? If yes, then content can be referenced as "from Ann on another site" - something like that. A link to a direct competitor, I generally remove.
Unfortunately, it's not black and white, we have to make judgment calls all day long about things like this. And, I don't necessarily speak for all Admin and I'm sure they'll jump in if they disagree or handle it differently.