Yeah, this is something I've been thinking about myself lately. I'll probably get back to it in another post but the whole "Mount" part of Mount & Blade seems underdone. Don't get me wrong, the combat part is excellent but seems there should be more to mounts in terms of personality, appearance, behavior and special accoutrements than there is currently. Your stable should be something of primary interest to you but to keep it interesting horses should be much more individualized and customizableable (whether in terms of saddles, stirrups, barding or other fittings).
I know that specific breeds of warhorses (Barbs via Iberia, Arabians via Norman Sicily and others) were maintained for certain traits. If the timescale of the game were exaggerated a bit the whole aspect of breeding could come into play as well. But just randomly generated (nameable) horses, tack and armor would be interesting enough.
The ability to control a more willful and aggressive horse could also become more of a gameplay issue than an abstract stat if equine behavior can be coded into the game. That would really help "personality" be expressed in a dramatic way. Some crazily spirited mount might just charge, or rear, or do whatever it wants and you'd have to control it. But it would take a spirited, aggressive, mount to charge directly at a foe or attack with hooves and bite (assuming this could be introduced at some point).
Anyhow, so much for a seperate post looks like it's all in this one.