Everyone seems to make valid points, but:
A great solo to me has musical shape. Musical shape in the sense of the player is using phrases and speaking in complete musical sentences. You don't necessasarily have to use big words (big chops), just make it make sense. Playing within the musical context is a major plus. It doesnt have to be a display of chops, it just has to make musical sense.
...that says a lot!
It seems to me, when you can play in such a way that you "bring the audience along with you", or you "connect" with the audience, that's when what you're doing on your instrument becomes "communication and/or interactive".
People like to be "surprised & caught off guard", that's why
syncopation works so well, because it's job is to do "
the unexpected". Therefore, when you speak a certain phrase on your instrument long enough for the people to understand it, RIGHT AT THAT MOMENT, is when you change up on 'em!
Then you pull them in again to something until they understand it, and SUDDENLY you switch up on 'em. And before you know it,
"you got 'em!" and just like McDonald's, they're LOVIN' IT! And they can't wait until it's over to show you how much they appreciated the "good interactive, conversation experience" you just gave them!
You find yourself using this "inanimate" object to speak to people, and stir up &
move people on the inside! You bring your instrument
alive, thus making for a more "meaningful & shared" experience. At least that seems to be the idea,
if you can pull it off. And that actually goes for
any instrument, or anything in front of an audience for that matter.
So, it would seem that,
"being able to effectively communicate with an audience" is high up on the list of what makes a great solo "great".
However,
having chops, certainly
helps you and
aids you in pulling all of this off a bit
better.