Before considering what is important to trading success, let's think of what is not important, today.
Many would-be traders believe that trading success is all about finding that one secret formula or system that explains and predicts price moves, and if only they could uncover this solution to market price behaviour, success would be assured.
The idea that trading success is tied to finding some specific ideal approach itself is misguided! Those seeking the one true answer to the markets haven't even gotten as far as asking the right question, let alone getting the right answer.
There are a million ways to make money in the markets, Unfortunately, they all are very difficult to find. But there are many, many ways to succeed. Some world class traders, like Jim Rogers, suceed using only fundamental analysis; others such as Marty Schwartz, succeed using only technical analysis; and still others use a combination of the two. Some traders succeed holding their positions for weeks, months, or even years, while others succeed on a time scale measured in minutes.
Markets succees is a matter of finding the methodology that is right for you - and it will be different for everyone - not a matter of finding the one true methodology.