IMHO, mounts in Guild Wars could be implemented within the current mechanics/balance of the game. They could be an attribute line in a new profession (perhaps along with lances). They could be mountable pets (beastmastery, lol) with the promised new controls. Or they could be mountable heros (with their own armor and weapons). In any event, mounted characters could have advantages for height (for archers and, perhaps, spear chucking and command), advantages/disadvantages for speed, power and/or offense/defense depending on the size/type of the mount. Balance issues would be similar to those of pets and heros, respectively. Although I do not recommend adapting their combat system, Mount & Blade demonstrates some of the animation and uses of mounts possible, if only with horses.

Wo Tan Ki, Ranger, Knights of Shadowpeak (KoS)