What do you mean by other forums, other boards?
"Off topic" can be rather hard to define, sometimes as some topics have many ramifications not to mention the odd word of greeting to someone you are familiar with or haven't heard from in awhile- speaking for myself I was off the forums for awhile but now I'm back and just renewing old acquaintances. I agree it's certainly in bad taste to type 10 pages about something completely unrelated. Sometimes as I've noticed in other forums you get situations where two or three people think they "own" the forum and make the rules. As far as I know all forums are public domain and open to anyone who is not abusive.
Nothing wrong with saying hello, how are you - I do the same since I tend to come and go.
Sometimes thread go off topic when they've run their course, too.
I agree that bringing a thread off-topic is extremely annoying and has an effect on the quality of the discussion about such a topic. That's why when I redesigned the forum, I added "write on wall" to the post. If you really want to greet somebody, somebody's wall is the perfect place. Alternatively, you can post a thread about something or somebody (if you have something nice to say) in the lounge.
Can one be off-topic in an "Off Topic" thread?
Now, suppose the off-topic thread were about off-topic posts. Could an off-topic post in the thread on off-topic posts in the off-topic category be off topic?
Whoops, i cut myself shaving, anyone have a topical dressing?