If the thread is not appropriate to the forum, then it may be seen as fluff or spam. On SitePoint, we have a dedicated thread where members can say "hello" and introduce themselves. Introductory posts made in other areas of the forums will be moved there.
Other forums may not want those kinds of posts at all. That's why it's very important, as ralph.m said in post #2, to read the forum rules and ensure you understand them before posting.