(Nick Reinking nick@twoevils.org):
Wiki syntax is a line-based syntax. There is /no/ wiki markup that spans lines. It makes editing much simpler: if you make a mistake and forget to close something, it gets closed off quickly. HTML is not designed to be human-editable; wiki syntax is.
I'm having a bit of trouble implementing the C parser because the Wikitext parser has a lot of quirks. For example, you say that no wiki markup spans lines, but if you take a look at: http://www.wikipedia.org/wiki/User:Marumari/Wikitext_Rendering_Quirks you can see that headers do span lines.
It's not easy, I agree. I wasn't aware that headers could span lines; I'm not sure whether or not I like that. I don't think so offhand. You'll also notice that I changed my mind about quotes--I think it's probably true that users will be somewhat surprized by forcing quotes to close on every line break, so in my long-range vision I closed them only at paragraph end.
My suggestion to an implementor is this: if the current code has quirks, it's largely because the expected behavior is undefined, so don't be afraid to define it yourself--if your definition makes sense, and doesn't screw up too many existing pages, it will likely be adopted.