Implementing a new mount is rather easy, you just need to design models/animation, that's like 1month's work at most.
But if you are making gears affecting your job abilities/TP/MP etc, there are just too many possible combinations, some of them are eventually going to be overpowered compared to others (in Endgame 8-man contents), and everyone playing that job is going to use 1-2 specific build. Then it just become another boring horizontal grind in addition to the vertical one we already have..
Diablo III's dev team spent several years trying to make their gear system interesting while balanced, but last time I heard it is still far from perfect. This can be a reference..
