Write a page, an article, an essay, or some similar block of thought, and put your name at the end so folks can see who wrote it. Signed articles invite comment, just by being on a wiki. Since a signature says "I wrote this", it would be best to leave the text of a signed article alone, and simply add comments to the bottom of the article, like a reviewer's notes. In other words, if you disagree with a signed article, state your disagreement rather than editing the article to make it agree with your view. Comments are usually signed, but not always.
An article with no signature, an anonymous article, is usually an idea that wants improvement. Anyone who comes along can edit the article, or even claim ownership by hanging a signature on it!
A "thread" is a series of connected comments, a running discussion. Threads might evolve from any kind of document, if people get into dicussing the contents of the originating document. To keep straight who said what, it's usually a good idea to sign contributions to a thread, or at least initial them like this -- ks
Sometimes a branch, or subtopic, of a thread seems to take on a life of its own. These subtopics might be happier if they were split off into a new page, with a link from (and back to!) the originating page.
The main idea is to communicate. If you have a better way of doing that than the examples you see on this or other wikis, try it out! Show your stuff. You could start a whole new trend of doing things a better way, and we'll all benefit from it.