Not learning all the notes on the fret board is the most significant obstacle to understanding how to play the guitar. Knowing what notes are in an A-minor arpeggio is fine and dandy, but if you don't know how to find those on the neck, it's just wasted information. Once you understand all the notes on the neck, everything opens up. Even if you love the way you play right now, learn all the notes on the neck anyway.

Start every day with a review of the notes A, B, C, D, E, F, and G on every string. In time you will learn them better and be able to use them more easily. Once you know what the name of the note is, you'll be more apt to analyze what you're doing. For example, why does the twelfth fret on the first string sound so good on an E blues? Because that note is an E, which is the keynote (which always sounds good). You may notice you're playing certain frets over and over again. How do those frets relate to the chord you're playing on? If you're not fret-board literate, you're flying blind. That's one of the reasons players don't achieve the potential of their guitar playing—they're blocked by what they don't know. So get in the know!