It is impossible to name a specific aspect of the game, knowledge or skill that exactly differentiates players up to 1500 from those above.
Chess played by 1500s is far from great and they still have a lot of room for chess improvement in all areas of the game.
Saying that understanding imbalances would do the trick falls short in many ways.
You can be an 1800 and still be relatively weak in some areas of the game (e.g. strategic understanding ("imbalances")) if you can compensate it e.g. with good calculation skills.
There are other players above 1500, that might have a solid strategic understanding, but still have some deficiencies when it comes to calculation compared to their peers.
Your playing strength is a reflection of the combination of all your chess-related strengths and weaknesses which differ from player to player.
Of course, for every chess playing strength, there are "minimum requirements" in each aspect of the game to be in that rating range.
You will not find a 2000 rated player who cannot calculate deeper than three moves and hangs pieces every other move or who is puzzled what to do in minor-piece-imbalance situations.
What you can say is, that on average, a player rated higher than another player is expected to be more proficient in each chess-related skill or knowledge, i.e. having a better chess understanding and calculating better.
They visualize better, calculate deeper, calculate more correctly, consider more appropriate candidate moves, have a better pattern recognition or intuition, evaluate better, know more typical plans, know more opening and endgame theory, have a better positional understanding and use better "thinking methods" and habits (e.g. asking themselves what the opponent's threat is before thinking about their own move, blunder-check their intended moves, briefly checking all promising checks, captures and threats, thinking while it's the opponents' move, adapting to time pressure, making practical decisions, etc.).
This enables them to play better moves, apply more pressure and blunder less.
Improving in any of these areas will likely improve your playing strength, however it may be benefecial to improve in areas first that are "holding you back", because improving in something where you already outclass your rating peers yields diminishing returns. For example, ceteris paribus, being able to calculate deeper will increase your playing strenght - as indicated by research on engines whose playing strenght increased by approx. 66 Elo per additional half-move calculated in advance.
I understand that you wished to hear "this is the one thing you have to do to finally break through 1500" or something similar after being excited learning about imbalances. But what it really is that you need to improve depends on how you play chess and your specific strengths and weaknesses, therefore noone can give you a "hidden" golden formula for how to reach a certain rating.
Chess players have been searching for an easy universal chess improvement formula for ages.
For you, it might be understanding imbalances. Or understanding how to better evaluate trades ("to take is a mistake"). Adopting a good thinking method improving your visualization or pattern recognition, or something else.
This is the reason playing longer time-control games and analysing them afterwards is so crucial to become a better player.
In contrast, there is a lot of evidence of what you should avoid if you want to improve your chess..
P.S.: It's not true that noone talks about imbalances. There are whole books dedicated only to this, e.g. "How to reassess your chess", and about every strategy book/course has a chapter on "good knight vs bad bishop" or asymmetric pawn structures and corresponding plans.