All of this is likely too general to be of much use.
Perhaps, but specifics differ from person to person and to provide a very accurate list is difficult, open to a lot of disagreement, would turn the post into an essay and not fulfil the objective I'm looking for. Knowing what roughly (rather just the usual even more vague, and perhaps inaccurate, study tactics and endgames) makes the difference between grades of players gives a starting point for things to work on. I believe that in almost all cases I can say to a 130 player who isn't an up and coming junior, particularly those who just play club chess the (perhaps a little arrogant sounding) I'm going to predict why you aren't going to win this game, and it'll be one of those reasons based on at least a hundred games OTB I've played. Sometimes a combination of several of the factors. It is unlikely to be for example because they couldn't handle an IQP position, missed opportunities to sacrifice a pawn for initiative or a better position, didn't know how to win Q vs R against good defence, didn't know a sharp line of the opening to 20+ moves, because they play the Scandinavian rather than the Sicilian, or couldn't calculate more than 3 moves ahead in a non-forcing line. These sort of things might (and do) decide games between much stronger players, but would be a waste of time for a weaker player of the grades I mentioned to put too much effort into.
If someone around say 190 (or 2100 to use the FIDE pool) could say that from experience that 160-180 (or 1800-2000) players fail to win games against them because of a list of reasons, that would give a starting point for evaluation of my own games to look for common patterns. Perhaps even solving just one of those problems might lead to a few more won games. So let's not dismiss something as likely to be not of much use without at least asking the question and seeing if anyone has an answer first.
Note: When replying to give observations please indicate both your (approximate) rating and the rating range you are commenting on so that the reply is useful to those looking to improve. Also please personal observations and not what you read from others unless it's something very specific and insightful. I'm interesting in long play games, so if it's an observation on a different type of game, please also state as the reasons may differ.
It's clear that (aside from playing 'underrated' junior players), most players a good step below my rating fail to win games (or let me draw games when I mess up) for usually the same reasons I've listed below. I'd be quite keen for someone say 2100 FIDE, to indicate whether there are common reasons they experience for such players in the say 1800-2000 range not winning games or letting them draw losing ones. In that way it gives some obvious things to work on to try to improve one's strength.
I'm ~160 ECF and commenting on players somewhere in the range 130-150 [my FIDE is lower at ~1850 and in that rating pool, so perhaps commenting on players 1600-1750]:
Opponent plays something 'safe' such as an exchange variation or London system. Not that there is a problem with those in a repertoire to avoid lots of learning, but it's often clear the opponent also doesn't know it as well as they should (especially what plans to adopt, what their and my objective are), plays stock moves and comes unstuck later. As that's their White opening they should know it better than I do.
Draws are when I mess up and save the game, not when the opponent is outplaying me. The opponent doesn't know how to build on or consolidate an advantage I've given them through poor positional play or not understanding the requirements of the position.
Their tactics are weak and they play often to avoid them in the earlier stages of the game. Often just by keeping going a tactical blunder may appear.
Their endgame knowledge is poor and is often why I save losing games or end up winning.
They don't sometimes consider what I'm trying to do. Yesterday my opponent fell into a basic trap: a 160+ player is unlikely to leave a piece en-prise unless in time trouble. Lack of tactics training or being able to calculate or visualise may be the cause.
Lack of trying to win (or hunger for a win) - see opening play. Perhaps the grade difference is scary. I make mistakes too, but unless I'm being set problems I'm not going to make many. This may be why 'underrated' juniors seem to play better than their grade.