Anything musical! For example: harmonically interesting tunes, tunes with tasty chords, great locked-in rhythms, loose rhythmic feels, interesting (foxy) melodies, soaring melodies, hook lines, groovy riffs, chord-melody styles.
And above all, improvisation. One of the most under rated skills in contemporary music making, (perhaps the only true 'music making'). A lot of people think this is just about taking solos but in reality improvisation is used on so many different levels and forms in both shaping music ready to play i.e working through chord voicings, lines and rhythms that work for the arrangements, and in the adjustments that take place between performers during live performance, e.g building the rhythm guitar part by changing chord voicings, textures and rhythm to support the singer or soloist.
Example: the chords for a tune may run: C, Am, F, G, which position do you play these chords in? - first position, open string or at 7th-5th fret? On the lower strings or higher string? Part chords or full chords? Full block chords or broken? Embellished or straight? Are there any colour tones added? - eg: added 9ths on I & IV chords? Would it work better with other chords substituted? - EG: Em for C, Dm for F. The choices are all yours to make as the song develops, on the fly, using your ear and experience to play what's going to happen next, often just a beat or two before it happens!