Using skills on mounts.

You should be able to cast certain non-combat utility spells and use utility skills while on a mount.

These skills absolutely should include: Light, Night Vision
These skills should be considered: Camouflage, Silent Movement, Vanish, Shadow Form, Dash, Sprint.
The later set should be nerfed while on a mount for obvious reasons.

Movement inability and reduction during casting of such skills should still apply.

Additionally instruments should be able to be used while riding, but you slow down to a slow walk while using them.

There should be the possibility of using various combat and support skills from a mount, but they all should take a penalty. There might be a "Horseback Combat" skill and a "Calvary Support" skill to reduce the penalty. Being hit by stronger attacks should knock you off your mount, and there should be a skill to reduce the chance of this.