I'll be helpful, as hard as it is to resist otherwise 
the answer is there is no 1 answer. a players strengths and weaknesses determine their rating..and since we are humans- and not robots- we all have different strengths and weaknesses. I can give you a cpl of short generalizations though- and thats all GNERALIZATIONS.
GENERALLY speaking, it is tactics that seperate the 1300 and 1800 players. I would take that a little deepr and say the higher up from 1300 you go, the more important a factor endgame understanding is. THEN, once you've mastered basic tactic recognition/attack formation, have a solid understanding what specific outcomes in the endgame equal either a win, loss or draw (if you dont know the winning positions of chess, how you supposed to reach those wins?) , and of course HOW TO EXECUTE AND CONVERT THOSE ENDGAMES, Well, now you should be at a higher level than 1800, and from here on up POSITIONAL UNDERSTANDING will become the ever increasing factor in determining the difference 0f strength. For example, an FM vs GM is likely to be determined by positional means, where as a 1600ish players will almost always be determined by tactics, atleast in my experience.
I know this is a superficial topic, but im interested in any comments for criticism on those general conclusions
Hmm... I'll have to remember not to get too frustrated then. It seems everyone has their kryptonite so to speak.