I've got a new one here - controlling your emotions.
Controlling your emotions while setting up a trade is one of the major aspects a very few people know and this thing can't literally be taught to any person as it needs to be conquered on our own. I believe that OP should compile a list of such opinions and put the best ones in the first post so to give a glimpse of the points that are needed to be taken into mind and learnt on our own before jiving into trading.
Another one is instinct. A good trader will have a strong instinct to have a premonition of something is about to happen. We all agree that knowledge and skills are important but our instinct are also important. Building a strong instinct takes a lot of experiences, it can't be taught. You can only learn it by a hard way - actually going through!