George Orwell said it (see, e.g., Politics and the English Language.)
And now there's good scientific evidence: using short words makes the reader perceive you as "smarter."
(In an added twist, now that modern PCs and printers have made every man, woman, child and dog their own typesetters, using simple, classic fonts was shown to be the smart play as well.)
Daniel Oppenheimer at Princeton University conducted five experiments manipulating the complexity of vocabulary or font style. Samples included graduate school applications, sociology dissertation abstracts, and translations of a work by Descartes.
Times New Roman, the default font for Internet text and writing programs like Microsoft Word, was contrasted by the italicized Juice font (the sort of font you might see in a homemade newsletter that's trying to be more than it is).
The simple writing done in the easy-to-read font tended to be rated as coming from a more intelligent author than the more complex drafts.
"Anything that makes a text hard to read and understand, such as unnecessarily long words or complicated fonts, will lower readers' evaluations of the text and its author," Oppenheimer said.
Simple Writing Makes You Look Smart (Yahoo! News)
Hat tip: Arroxane and Ryan on the TECHWR-L mailing list.