Well for me i look for a good theme color that is'nt too dark or over powering bright. second it has to have a good seperation of member info from the reply..
in my post , notice the two seperate colors of grey to contrast them both..
next i look for a good solid layout of the site, and not cluttered with windows, i personally don't like split windows on the main web page.
the site must have a good sence of flow: example
1)easy to read text
2)a limit on how big the Avitars can be, is a must.
3)a limit as to if or how large a member's sig can be..
these are just some examples that i hope can help..