I always notice how people keep their shoes, it's a good indicator. Also, if they wear a belt or not and if it matches the shoes. They may seem like little things but can point to larger and more problematic ones. The same with how people keep their car looking, inside and out.

We get first impressions wrong for many reasons, prejudice for one, ignorance and arrogance. We all do it. The good thing is that we have the ability to change as we learn more about a person and the smart ones are doing so, like you did with your mate. The idiots hold to their false impressions and miss out on potentially great relationships/friendships/colleagues because of it.