One of the worst things that can happen to a wiki is if it descends into dictionary mode. The only plausibly decent wiki-as-dictionary sites are those which specifically set out with the aim of being a dictionary, and even then they don't tend to do so well at it. Consider for example wiktionary.org, a spin-off of Wikipedia; it's nowhere near as popular as Wikipedia itself because it doesn't give people as free a reign over the content. You just don't get to be as descriptive and all-encompassing in a dictionary, so it ends up not being as good. Another problem with wiktionary specifically is that it's a *spin-off* of Wikipedia, so people don't go to it as they see it as an inferior sister-site.
Wikipedia itself is, in fact, rather limiting in what it allows you to do. The Shakespeare page, for example, only gives an absolute smattering of the possible information that could be provided about him, mainly because of the compactness requirement of an encyclopaedia. So what ends up happening is that wikis-as-dictionaries that aren't specifically meant to be dictionaries go somewhere in between; they start defining stuff in very odd terms.
For example, Aaron Swartz's logicerror site was intended to be a rather encyclopaedic site, but tended towards the wiki-as-dictionary phenomenon. Aaron has since become a semi-regular contributor to Wikipedia instead, incidentally. If you look at logicerror's uri page, you'll see a good example of what I mean: instead of providing a large encyclopaedic defintion of a URI (compare wikipedia:URI, which is quite comprehensive), it gives a very general datum about it ("The Uniform, Resource*, Identifier* (URI) is the cornerstone of the Web.") and then proceeds to talk somewhat about peripherally related resources; it doesn't even link to RFC 2396, which was the URI specification of the time—now superceded by RFC 3986, giving URI buffs a whole new number to remember.
Of course, there's nothing particularly wrong with that. It's a very personal view of what a URI is, and it's information that is most relevant to Aaron in some respects. But that's the point: a wiki-as-dictionary generally only serves the writer, and often the requirement changes and the wiki-as-dictionary fails as a result. Moreover, it rarely succeeds in being as useful to others as a well-groomed wiki.
I think, incidentally, that "wiki" is one of the least phenomic words that there is, which is a bit counter-intuitive really when you look at the phonemes. It's just something about the w and k placement which makes it seem so vulgar. Anyway, that's why this page is called notadict, with a slight nod to notabug, instead of wiki-as-dictionary or something possibly more appropriate.