We can do psychological testing to determine when simplicity switches, but other people will switch it at slightly different times. Best all around to allow both methods.
This I have to disagree with totally. Leaving in a mix of both is horrible. Our markup here serves a different purpose than other markup languages. It /must/ be editable by novices. Wikitext that produces a nice page, but can't be edited, is bad wikitext. For an extreme example, see the article "Duesseldorf"--this is precisely the kind of abomination that allowing too much HTML produces. It might be a fine article, but it will never be improved because it's impossible to edit. That's fine for web page, but not for a wiki.
As comfortable as all of us are with HTML, /we/ aren't the kind of people we want editing articles (except maybe those on computer subjects). We want Bridge players writing about Bridge, and cat breeders writing about cats, and campers writing about camping--the kind of people who have never even heard of HTML are the kind of people we want most to attract and make use of. We computer nerds are used to dealing with special syntaxes; it is we who should adapt to them, not the other way around.