I know what you mean when you can't define “professional“ and I myself am at a loss as to what makes a site “professional” and one that isn't.
Your guidelines are good, but I think it has less to do with fixed width boxes and more just liberal use of whitespace and fonts that don't leap out at you. You don't have to necessarily fill the space but it does have to … flow nicely.
Now, out of these three sites
http://www.conman.org/
http://www.conman.org/people/spc/
http://boston.conman.org/
which looks more professional, and why? What about:
http://www.siteremedy.com/
Actually I think the third link is the most professinal...too much whitespace is probably worse than too little, whitespace usually should be used for border for the entire content...something like that.
Wow? My blog has a professional design? Wow …