Things like responding by quoting an earlier comment but adding no original text, simply to 'highlight' it or to emphasize that you agree with it.
Leaving flame-bait statements and never responding to any of the replies.
Deliberately pretending not to understand a topic, or attempting to draw out conversation in an effort to increase your post count (which can be painfully obvious).
Posting over 10 times in a thread with less than 40 replies. I mean, it's almost like having a conversation with yourself.
Leaving very short replies, in the region of two words. If it's a joke, sure that's fine. Or occasionally doing it is fine too, but frequently doing it = spam.
Many of the bigger threads on this site are full of spam, like the game threads such as say something about poster X above, or tell a story. Some actually contribute to the thread, and I see no problem with such threads, but if you browse through them, there's hundreds of replies from the same users, who spent mere seconds writing the first thing that pops into their head to increase their post count.
I suppose not all the above count as spam, but it sure is annoying.
There seems to be a lot of threads about post counts and such arising lately...
