This is an interesting question and one which is open to debate, but personally I think there are a few things which are essential to become a good player.
Firstly I think that having a good temperament is a must, as the very nature of the game is such that you will have to deal with bad beats without becoming irritated or upset and allowing this to affect your play.
Also, having a reasonable grasp of maths is important. You need to understand odds and percentages and be able to calculate them fairly quickly to reach the best decision in any given dilemma.
Most of the great players over the past few decades have all had this ability to deal with bad luck, and been able to quickly work out their chances of hitting a hand. If you do not feel that you fall into this category just yet, then do not worry, because the more you play poker, the more you are training these skills. Aside from the important skills I have mentioned, practice and experience play a large part, and natural skill I feel is also present in some players more than others. Take for instance Stu Ungar, as one of the greatest ever Holdem players, he won the WSOP Main Event for the first time having mastered a different game to the point where players refused to sit down and play it with him. He had only been playing Holdem a short time when he won the event, and went to win it twice more before he tragically died in 1998.
Phil Hellmuth is the great modern day champion, having won 11 bracelets in total and legendary status within the game. Both these men and other talented players everywhere, have many strengths and aspects of their game that help them succeed, but practice is always what will hone these skills ad help them to become the best they can be.