1. String crossing
Moving the bow from string to string can be daunting, particularly in faster passages. It involves a new orientation of the bow arm (and of the left hand fingers as well). Fingerings and shifting often are used to solve this problem, but sometimes the string crossing simply have to happen. In certain circumstances, this string crossing becomes an important part of the music, as often seen in Baroque music.
2. Bowing notes of uneven durations
Long notes require longer, slower bows while short notes require shorter, faster bows. But what the arm does easiest is bows of equal speed and length. String players frequently solve this problem by hooking short notes into long notes, but this is not always musically possible. Another solution is to balance bow speed with bow pressure, but this is much more difficult than hooking notes.
3. Bow placement on the strings
The location of the bow as it touches a string is vital to the type of sound coming out of the instrument. Different volumes, colors, and intensities are achieved by playing near the frog, the middle, and the tip of the bow, as well as placing the bow near the bridge, the fingerboard, or in the middle. Solutions to this problem are even more difficult than intonation, but ultimately needs to become instinctual for the performer.
4. Pitch production without specific finger placements
With the exception of trombones, all other instruments have keys, holes, valves, or frets to indicate where pitches are located. While some of these instruments require some pitch adjustments with embouchure, no other instruments have such a random system to create pitch. Good intonation is an issue for all players and no one has come up with a satisfactory, guaranteed solution to it.