There are few problems with open source dictionaries as served by dict.org. It is also the default source of dictionary app in Linux.
It requires the singular form to find the right word. This is a major pain. For example, if you lookup “chairs”, the result is this:
No definitions found for "chairs", perhaps you mean: gcide: chais Chair chains wn: chair chains moby-thes: chair
When you lookup a word with accented letters, e.g. passé, touché, précis, ménage à trois, lycée, raison d'être, … 〔☛ English Vocabulary: Foreign Words〕, the dictionary gives you a error “No definitions found”. You have to use a form without those special characters.
No definitions found for "passé"
See also: Diacritics: Trema, Umlaut, Macron, Circumflex, and All That.
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 the phonetics for “understand”:
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 online dictionaries 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 (), dict.org hasn't improved a bit. All of the above problems are still there.
On the other hand, the open source http://www.wiktionary.org/ is pretty good now.