The purpose of taking a solo should be that of playing any music: communication. To paraphrase Keith Jarrett, it is the process of taking the idea in your mind and putting it in someone else’s mind by the mechanism of playing a note. Listeners don’t care what you play, they care about how you make them feel.
It is good to be able to play a wide variety of ideas at a wide variety of tempos. This makes it less likely that a listener will feel bored. Boredom defeats the whole purpose of playing in the first place.
Harmony can be used to inform the ideas that we communicate with. To have variety, it is good to know how to improvise a melodic line from point A in a chord progression to point B in the progression in different ways. Knowledge of a variety of harmonic devices provides an important means to create these paths.
Technique is the thing that allows you to achieve this variety of expression. If you can’t play something, it doesn’t matter that you thought it. Technique is not an end unto itself however. Technique is about rhythm, not about playing “fast”. If you have technical skill (chops), you have the possibility of subdividing whatever metric unit you are dealing with when improvising. At faster tempos this necessity becomes even more acute, since you’d be hard pressed to create an interesting solo with nothing but half notes and whole notes at your disposal.
For guitarists, developing rhythmic fluency with scales is a different proposition than doing so with arpeggios or intervallic structures, from a technical standpoint. To improvise effectively, you need to be able to draw upon all of these things.
Phrasing is the difference between communication and a collection of notes. One thing that guitarists can do is play continuously without ever stopping to leave some space so that the listener can process what they just played before being bombarded by the next idea which won’t be processed no matter how cool it was because they didn’t have any time to do so not to mention the fact that right about now the rhythm section is starting to feel like a Jamey Aebersold play along record not other individuals who are involved in the process of creating music with you is this getting hard to read yet oh I get it…
Put another way, leave some space. Let the listener process what you just played. Let the rhythm section get into the mix and contribute ideas. Learn the dramatic pause.
Contrast is a concept that be applied to a solo in several different ways:
Fill up space (play a lot) — Leave space (rest)
Play in the upper register — play in the lower register
Complex ideas — simple ideas.
Loud — Soft
These concepts don’t really need explanation — you could apply them the next time you play.
Singing is the thing that lets you know what you really know. The ideas that you can sing, when you’re not holding the guitar, are internalized, not those that your fingers know but your ears haven’t learned yet. When we are learning something new, our hands are usually ahead of our ears in that we can execute something on the guitar before we have internalized the sound. Singing the new things that you’re learning on the guitar will help integrate them into your improvising much faster.