One of the most important parameters for long-term stability:
How much collateral is required to enter a short position (this number is hard-coded at 2x in Bitshares XT).
This one's obviously really important; if more collateral is required, the less likely it is that the system will collapse 2008-real-estate-market-style. On the other hand, if less collateral is required, it will be possible for more BitAssets to exist and this presumably should spur greater economic activity.