Some dictionary definitions do give "lurk" a hostile or sinister connotation, but you're probably right, Wanda. It's a matter of semantics.
I like your description of different types of lurkers. I'd also add what I call the "distant" lurker - someone who mostly skims the titles of a newsgroup or mailing list, and only infrequently reads the messages. Or someone who hardly looks at the posts to a mailing list at all, but still decides NOT to unsubscribe.