The general criterion of starting in with confidence is not really reliable at all.
I think most new contributors with experience in other media would look around WP a good deal before they started editing. I certainly did, not wanting to appear like a utter newbie. The general criterion of knowing what to do when you come here is not really that reliable. Even the use of abbreviations can be picked up fairly easily; other forms of writing on the web use similar methods, and its just necessary to learn a few new ones. The sort of markup used in WP is very simple--a great many people know this already--I found my problem was to unlearn the parts of html that weren't implemented.
And I know many young teenagers who can catch on to this sort of thing a lot faster than i did.
David Goodman, Ph.D, M.L.S.