I have developed a lot of UI's. Everyone has their own opinion and it is very subjective. When everyone sees a nice UI they know it but getting there is very opinionated. Well here is a least one objective thing: Speed. It is a pet peeve of mine that people say something is too slow but when I ask them for the requirement of time they don't have one. Here is a law (Weber's Law - http://www.usd.edu/psyc301/WebersLaw.htm) – That states basically if you add more than 20% of time to a UI then the end user will “see” it or perceive it as slower.