Similar to how we have the alliance and horde symbols in the list. I think it would be nicer to have a few more icons in the list to differentiate mounts. Specifically talking about having the class icons next to the legion mounts, paladin chargers, dk deathcharger and bone gryphon plus the warlock felsteeds. right now they're red/greyed out on classes that can't use them.

That's a great idea. This could also work with professions.
There is an overlap though. Some paladin mounts are restricted to a race and therefor to a faction as well. Currently the faction icon is already shown there. Shall we display the faction, the class or maybe both?
(Examples are: Thalassian Charger, Exarch's Elekk)

Also, covenant locked mounts with their icons!
I just had this come to mind. I'm glad you think it's a great idea, I think it would be a great addition.

Finding proper icons is not so easy. I've added some icons for classes and covenants. I'm not fully satisfied with the class icons though :-/
Anyway they are added in version 2.12. Hope you like it :)