Xah Lee, ,
There are few problems with open source dictionaries as served by dict.org.
It requires the singular form to find the right word. (e.g. try lookup “chairs”) This is a major pain.
When you lookup a word with accented letters, e.g. touché, précis, ménage à trois, lycée, passé, raison d'être, … , the dictionary gives you a error “No definitions found”. You have to use a form without those special characters.
See also: Diacritics: Trema, Umlaut, Macron, Circumflex, and All That ◇ English Vocabulary: Foreign Words.
It uses some idiosyncratic made-up pronunciation system (typical of American dicts) as opposed to IPA. Worse is that ASCII characters are used to emulate pronunciation symbols, rendering it unusable. For example, compare:
[u^]n`d[~e]r*st[a^]nd"
ˈəndərˌstønd
The first is from dict.org, the second from New Oxford American dict. (For a comparison of major US dictionary's pronunciation systems, see: English Phonetics: IPA vs American Heritage Dictionary vs Merriam-Webster.)
Most commercial dicts online have voice-recorded pronunciations, of course. This is particular useful for borrowed words, e.g. those with accent marks.
Often, there are 2 or more results from “The Collaborative International Dictionary of English”, apparently of the same version but they differ slightly in content. e.g. lookup “precis”, then it gives:
From The Collaborative International Dictionary of English v.0.48 [gcide]: Pr'ecis \Pr['e]`cis"\ (pr[asl]`s[=e]"), n. [F. See Precise.] A concise or abridged statement or view; an abstract; a summary. [1913 Webster] From The Collaborative International Dictionary of English v.0.48 [gcide]: precis \precis\ v. t. To make a precis of. [WordNet 1.5]
The definition quality, vocabulary size, up-to-date quality, are inferior to commercial dictionaries.
As of today, the open source dictionary dict.org hasn't improved a bit. All of the above problems are still there.