The ability to solve problems. Every position is a problem of sorts, not necessarily a tactical or play and win or draw, but rather something that requires proficiency in solving. If you have no forced way to obtain an advantage then you have a strategic problem to solve: find the correct plan, then find the best way of bringing that about. This involves calculation since you wouldn't want to lose a piece merely by carelessly playing ...Ne4 ...f5 and ...Ndf6 (optimizing central control and maximizing coordination) or similar schemes right? Or if something would otherwise be excellent but result in a refutation (can't play 1.f5 with the idea of opening lines towards the king and 2.f6+ because the exchange sac 1...Rxe4! makes me lose a piece after 2.Rxe4,Bxf1) then at least that one move needs to be scrapped.
What actually makes a player better than another in chess?
Fischer believed that chess had reached a dead end because of the need at the top levels to memorize so much opening theory. That's why he pushed 960 Chess (also known as Fischer Random Chess) so hard.
But as a player Fischer memorized tons of material. It was one of his strengths. Before his match with Spassky, he tossed a book of Spassky's collected games to a journalist and told him to pick any game. The journalist did so. Fischer closed his eyes and reeled off all the moves.
Fischer believed that chess had reached a dead end because of the need at the top levels to memorize so much opening theory.
Thank the chess gods for Magnus.
Knowledge of what matters most, according to Andy Soltis. For example, the strong players realize they can accept positional weaknesses for something good enough in return, like the bishop pair.
Fixing: I think I know what you're saying. I too am impressed by Magnus's relative lack of concern for opening preparation.
A few weeks ago I was looking at a Magnus game in which he played something like a Grand Prix Sicilian in reverse to 1.c4. It didn't even seem to be a surprise weapon he had cooked up (though may be it was). It just seemed like he was saying, "I don't care about the opening. Let's play chess and I will beat you."
I think that too much play without study hold you back...You will tend to follow your own old patterns and not have time to develop a different, correct thinking process, and to learn proper strategy and new ideas.
It all rides on the " Blunders "
Understanding...
Understanding the position.
IQ
Or beer! 
Vague question, Sabrina444.
I take it you're doing some soul-searching in your chess, otherwise the discussion on this thread is simply academic, which does not a stronger player make, practically. Long lists of chess 'traits', or orange juice, won't help either. So, Sabrina four four four, what do you want, exactly?
Two parts:
1) knowledge (openings, tactics, endgames, strategy, etc)
2) practical skills (calculation, judgement, time management, etc)
I like that answer. Wouldn't tactics fall under practical skills since they involve judgement and calculation? Here's a tactic I calculated but didn't play because I saw a nasty check that loses material and hands white the initiative at the end:
Starting out in chess as a junior, I think having a GM coach or going to some Russian or St Louis chess academy is what makes one a better player. Too many amateurs are self taught and will never reach even the lowest master level.
Two parts:
1) knowledge (openings, tactics, endgames, strategy, etc)
2) practical skills (calculation, judgement, time management, etc)
I would simplify even further and say you can group it all under one category:
1) Mistakes
One player is better than another because they make fewer mistakes on average. Mistakes can come from one player having less knowledge such as endgame knowledge, or less skills like calculation ability, or from psychological factors such as ability to stay focused or handle stress. Also degrees of mistakes matter as you play stronger opponents. Missing a mate in 1 costs you the game even against bad players, but giving your opponent a slight space advantage is not likely to cost you the game unless you are playing a very strong master.
Many of the great players have been known for their outstanding memorization of chess lines and thousands of games played.
Recent ones are: Kasparov, Carlsen, Fischer and many other.
Sorry Rickett2222 but Fischer was enemy of memorization and this led him to dangerous disease also.
In which sense was he an ennemy of memorization. For the other players he played against as when he said if you memorize the games that i lost i will play differently the next time.
He actually was recogniized for his tremendous memorization of several languages, Russian and Icelandic, German and a few more.
His recall of games played was simply phenomenal.