I look for design mostly. I think a theme needs to stand out from the crowd (and there are a lot of blogs now), and I also try to pick themes that won't date too quickly. My skills lie in coding so I know I can change the backend quite easily if I need to, but my graphical skills are not so hot!
When you say it doesn't date too quickly, do you mean the style of the design? Such as grunge/minimal/corporate/ etc. ?