Of course the most difficult thing on the violin is always intonation. The second one is rhythm. If you play in tune, in time with a good sound that's already high level. Those three are the main things.
Ruggiero RicciEvery violinist has a different style, so it's important to be able to recognise their styles. You don't have to like everyone's style but you have to know these styles.
Ruggiero Ricci