Ok, being a guy contemplating getting an HPA tank again after literally a 2.5 year haiatus, excluding three distinct days of pump play and a day of airsoft while overseas, there are three things that may convince me to get something other than Ninja.
Rock-steady set pressures. Within 15psi, over and over again, every time, from 4500psi on the gauge down to 900psi on the gauge.
Fast recharge. Letting a Mag R/T through a hopper without shootdown should be good enough.
User-rebuildable. Duh. A high-pressure and a low-pressure piston (which miiight be his plan, based on the scant information that there are no shims to swap) would be one of many good methods to keep it simple but adjustable, but then precision is left to the springs, which isn't great for selling every tank at the same set pressure. Mostly, though, that thing better have a standard fill nipple, standard tools should take it apart, and o-rings should be readily available. And I swear, if there's any green loctite, I will never buy one. All regulators should be airport-friendly as a standard option.
Brass or SS threads. Anodized aluminum bonnets should be illegal. Poorly plated threads should warrant a full refund.
Under 3" long, bottle to tip. Anything more is excessive.
Nice features would be an output pressure gauge, PTFE o-rings standard, shorter overall length, externally adjustable output pressure. Anything else is hype or too complicated for such a crucial part of my gun.