Mine would be:

Learning AL from brushing, etc.

Dropping the barriers to most hunting grounds!

Being able to ride in groups

Being able to default to MY horse when in a crowd. It's darned irritating to have to find the fourth silver horse or third Akhaal horse....especially at a joust when there's a crowd of horses (which is so wonderful to see!)

Being able to ride double, perhaps just at a walk or trot..

Having horses be able to find their way to X like caravans do

Having the horse demonstrate our bond by nosing me, or nickering at me - making it a bit more personal! ;)

None of these are new things, and they've been expressed so much better than I am doing at this point, but I wanted to throw those out there as things which really matter to me as a horse owner.

player of Jaihala, rider of Arinen
player of Kilaana, rider of Honey
player of Varrisa, rider of Bonfire