The biggest thing is understanding instrument depth/richness as well as sound projection. I will compare this to listening to the radio in your car. It is much more enjoyable to listen to the radio when you have very good bass and nice treble. It makes you want to turn up the sound as it is soothing on the ears. Instrument depth/richness refers to the bass and quality of sound you are hearing in this situation. Instrument projection on the other hand, refers to the positive loudness of music, which is very enjoyable when the quality of the sound is good. So the best case scenario, you find a violin that has both of these qualities, which are very easy to identify when you compare one next to another.