As Claude Debussy said "Music is the space between the notes." This, I find to be such an important principle, that many of us aspiring musicians tend to forget. Sure we can practice playing such technically difficult stuff, but it all falls down when we misjudge the value of notes.

I myself find it tremendously difficult to accurately attack and release notes. The key signature and note values given on paper are not the only key that determines the length of each note. It's also an internal feel, that one needs to acquire to master this technique.

A problem that I've recently noticed amongst my peers, is the tendency to rush quarter notes and half notes when they're surrounded by a series of eighth & sixteenth notes. As one of my my teachers always said, "I want the full dollar. Right now you're giving me about 90 cents, but I want the full dollar."