Writing to be read

Tests have shown that a sentence of eight words is very easy to read; of 11 words, easy; of 14 words, fairly easy; of 17 words, standard; of 21 words, fairly difficult; of 25 words, difficult; of 29 or more words, very difficult; so this sentence with 54 words, counting numbers, is ranked impossible.”

The 50 most frequently looked-up words by readers of The New York Times (1/1/2010 through 5/26/2010)(Alphabetically, not by number of look-ups)
 
alacrity — antediluvian — apoplectic — apostates — atavistic — austerity — baldenfreude — canard — chimera — comity — crèches — cynosure — démarche — desultory — egregious — epistemic — ersatz — feckless — hegemony — hubris — incendiary — inchoate — Internecine — jejune — Kristallnacht — laconic — Manichean — mirabile dictu — nascent — obduracy — obstreperous — omertà — opprobrium — overhaul — peripatetic — polemicist — prescient — profligacy — profligate — provenance — putative — redoubtable — renminbi — sanguine — sclerotic — solipsistic — soporific — sui generis — ubiquitous — verisimilitude

If your reader has to look up a word or more they begin to be disconnected to the content; they'll have to read the sentence more than once if they return from looking up the word. And they may think you're speaking over their head and that your content is not meant for them.