For the most part good wikitext dialects do a pretty good job of letting you write formatting stuff in a way that looks and feels very natural. Markdown is an excellent example here; much of its formatting looks basically how you'd write it in a plain text document such as a plaintext README or email message (and this is by deliberate design). I will semi-modestly claim that DWikiText (my wikitext dialect here in DWiki) does likewise, again by design, although it probably isn't quite as good here as Markdown is.

But there is one exception to this. Namely, I've yet to see a natural looking syntax for tables in wikitext. The qualification is important because there sort of is a natural syntax for tables in plain text; if you need to present such a table you generally line up the columns with enough whitespace between them to make it plain that this is a table and maybe indent the whole thing a bit to make it stand out from the regular flow of text. The problem is that this syntax is relatively obvious to humans but generally is terribly ambiguous and hard to process for computers.

Past that I've seen a number of syntaxes that look semi-okay, generally involving drawing explicitly table and cell borders in some way, but they all seem relatively awkward and unnatural to me (this includes the syntax in DWikiText). None of them look like something that I'd put in ordinary plain text and that would be immediately obvious to a reader. Possibly the problem is simply too hard and the real solution is some form of formatting that's not natural but is clear and easy to read and write.

(I suspect that one doesn't want to borrow, eg, the tbl syntax. It may have seen a lot of use but I don't think it's exactly the clearest thing to read or write.)