"When people rate a site as aesthetically pleasing they are responding to two main factors: that it is orderly, and that it is just a little bit unique:
Lavie & Tractinsky, Assessing dimensions of perceived visual aesthetics of web sites. Intl Journal of Human-Computer Studies. 60 (2004) 269-298."
Thank you to Susan Weinschenk, the "Brain Lady" for pointing out this interesting article. Her website (fascinating info), is What Makes Them Click.
The original article is here: http://www.kommune.skoleintra.dk/Infoweb/indhold/Forside/10.1.1.101.8129.pdf.
Also see her "100 Things You Should Know About People"