I am going to write a massive essay on this.
I hope you are prepared.
edit: okay it's not that massive probably but I hope you like it anyhow.
First, there are a LOT of different skills you can master in this game. Trying to say "it's X that makes you great" is really hard to pinpoint, because there are so many other things that will impact how useful that trait is. When you're at a lower level that method of improvement works because every player has SO MUCH to improve on, they have no real strengths. Just by developing a strength, you stand out.
That in itself is the key to seeing yourself ascend. The prime difference between players who see lots of success and players who flounder--despite showing lots of promise--is their ability to focus the match on their strengths.
If you have unbelievable shield pressure, and you are good at forcing people into their shield, you will see success. If your spacing is incredible, so you play a character that emphasizes your spacing, you will see success. If you are good at utilizing lots of knowledge and tricks, and you play a character with many secrets and gimmicks, you will see success. Good shield-grabbing and edgeguards? Play near the edge and enjoy making it out of pools.
This is why you sometimes see players who do really well and show promise, but randomly fail against other people, or get absolutely stomped by somebody who's actually just a little bit above them. The situations they find themselves in don't play to their strengths.
So obviously, more strengths means adding more dimensions to your game. But the thing is, it almost doesn't matter what you pick, as long as you have the ability to keep the game focused on those dimensions. If there is a giant gaping flaw in your game--you suck at executing from your shield, for instance--and you play a character that spends a lot of time there, or your most common/worst matchups involve acting from your shield a lot, you are going to have serious issues advancing. So your choices are 1) always avoid the situations you suck at or 2) get better at them. In tournament, I recommend picking option 1 every time. In friendlies, I mostly would recommend option 2. In the time period right before a tournament though, I recommend going back to 1 though.
/thoughts