I think for me the top 10 looks something like
1. Jackson's regression
2. PG play
3. The vets being near "unplayable"
4. Booker's defensive effort
5. Ayton's rim protection
6. Bridges not getting more mins
7. Warren not getting more mins
8. James Jones/Sarver making a panic trade by overpaying
9. Suns scheme leading to way too many turnovers
10. Fastbreak defense
This is actually a really interesting topic. Let me take a stab at it too.
1. Turnovers.
2. Josh Jackson. Everything about him. He is this high on my list because he is so damn talented.
3. Point Guard situation.
4. Defense (related to turnovers.)
5. Three-point shooting (very much related to the veterans on the team as well as JJ.)
6. Booker flat-out refusing to play defense.
The rest... not significant. If the Suns are patient and don't panic, things will sort themselves out. Anderson and Ariza will pick it up, Warren and Bridges will get more time as the season progresses, and Anderson, Ariza, and Chandler will be gone next year, Ayton will only keep getting better.