One of the most important attributes of any great singer or instrumentalist is great timing and phrasing. This means they have developed the ability to begin and end a note or chord at precisely the right moment and to fit their playing in with the arrangement for maximum musical and dramatic effect. To develop a good time feel right from the start, you should always practice with a metronome or drum machine if you are not playing with other musicians.
Great players also mean every note, nothing is unconvincing or half-hearted. A good example of this is the bass player in a Jazz group. Walking basslines often consist mainly of "simple” quarter notes. A great player can drive the whole group with the way they play these quarter notes. It is the feeling and attitude the notes are played with that makes all the difference. Once you can play something, you must be totally committed to it. Never think "this is easy, I don't really need to concentrate here' or this attitude will become a part of your playing. Good players make simple things sound great, and this is what you should strive for right from the start. Treat everything you play as music, rather than having one attitude for "exercises" and another for "real music".