piskie's right. Human factors research is clear that all caps is harder to read. The ascenders and descenders of lower-case letters increase differentiation and facilitate recognition. I've seen mention of a 10% reduction in legibility when all caps are used.
Both for the professionalism and readability mentioned previously.
Furthering the ascenders and descenders noted above... The eye actually reads based more on the outline of the words it sees than the letters contained in the words, which is one reason spelling mistakes often go unnoticed.
I hvae raed taht in most words huamn eye olny needs the frsit and the last letter in hte snenecte to be correct and most vowels on the right place, if other lettres are minlged up the brain can still make sesne of what is wirtten, epsecially with logner wodrs. ;-)
There was a better example of this, but I cannot find it.