Discover the Factors that Separate Good Harmonica Players from just Average Players

What separates a good harmonica player from "just a player"? 

A good harmonica player has got a musical sense of what they're doing. 

There's a bad syndrome among harmonica players here in the US of playing in isolation and playing and thinking of themselves as a soloist without really having the background for being a soloist. 
If you take a course on soloing, in Jazz for example, the very first thing you do is learn chord structures and the arpeggiated notes of those chords and how the chord progressions work throughout songs and how the notes you then play relate to those chords and those chord progressions,
and that's way more than the average harmonica player ever gets into. 

But, not to despair, it's not that difficult to do these things if you are sticking to the 12 bar blues progression, which is very predictable. 
So if you learn the the chord progressions for the 12 bar blues then you are halfway home right there, but that's just part of being a good player, being a good soloist, being a good harmonica player

-  there are really so many aspects.

Another major problem is most harmonica players overplay

- they try to dominate the song by being too loud and end up not fitting in, and they also overplay by playing too many notes,  at least right at the start.  There are places in songs where it's appropriate to play tons of notes, but the average harmonica player just come on too strong and ends up sounding amateurish instead of impressive. 

Another major thing that a good harmonica player has that an average player doesn't, and that is a good harmonica player has good tone. 

Good tone is not that hard to get on the harmonica - it's really important to think in terms of air space inside your mouth

-  that's the most important place to get good tone: keep a nice big open space by dropping your jaw, dropping your tongue when you play.  

So you have a sense of musicality, a sense of the chord structure, sense of which note fits in where,  and you have a sense of fitting in with the song and with the band instead of overplaying, and you have a sense of the tone of your playing
- all those things are very important. 

There are many more things involved in being a good harmonica player; with any instrument you've got to have good rhythm (which can be developed) have a sense of emotion, sense of projecting your personality through your playing, a love of the style of music,  a love of the song, and really it goes on and on from there.