Timing. Rhythm and timing are the
most important elements of music. More important than the notes. A wrong note at the right time is better than a right note at the wrong time. (Of course, a right note at the right time is best of all...
)
It took me many years to realise that timing was my weak area. Not only imprecise placement of notes relative to a beat, but a tendency to speed up when improvising, especially when playing live. That is, I kind of knew I had a problem there, but was lazy about addressing it. I thought I could control it, didn't realise how embedded it was.
With everything else, even though I was self-taught, I was pretty much lucky.. Just accidentally, I happened to give the right things more importance, and never really got bogged down in the wrong things.