The way I prefer to look at it ,being a able to play scales and arpeggios fast,doesn't add up to having chops,that sort of skill tends to lead to so called improv that is merely playing fast permutations of learned patterns over a given rhythmic,chordal framework.Which can of course be really fun to do. But if carpenters showed off their skills the way some musicians do ,all the nails would show and there'd be afuckofalotof 'em The improv skills that I seldom hear discussed are Composition skills,which include well develpoed sense of beginning/middle/end,the ability to tell what is essential and what is embellishment , Daring, problem solving( follows right on the heels of daring) effective use of Space.The ability to work effectively with line,harmony,color,texture, blend/contrast motion,repetition/variation foreground/background,tension/resolution ,motivic development,dynamics, timing,counterpoint,gesture etc. as creative tools. Most of these compositional elements can be employed working with raw sound as well as tuned instruments. An interesting approach to group improv can be to agree in advance on a story,and then try to tell it . It seems to me that improvisers who pay attention to each other should be able to play together regardless of differences in instruments, styles, techniques and musical knowledge, by bringing big ears to the session and responding at some level to the essence of what each other is doing.