With all these RPG elements of gear and levels, I think it is extremely important that Destiny features non-linear upgrades and equipment. What I mean by that, is you shouldn't be finding better gear that replaces what you already have, but rather different options.
Lets take a leg equipment for example. Instead of having Greaves +10 armour and Greaves +50 armour, where one is clearly better, it should be along the lines of.
Greaves +10 Movement Speed & +5 Armour
vs
Greaves +20 Armour & -10 Movement Speed
Now one is not necessarily better, but rather different, and perhaps more fitting to your playstyle.
As I understand it, all gear/equipment is persistent in multiplayer and singleplayer/co-op so having equipment that is different but equal is the way to go, unless we are separated by our levels and expected gear, which would be very lame.
Or everything could simply be aesthetic changes.
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I would MUCH prefer non-linear upgrades. Obviously you will find better and better gear, this cannot be avoided. But they must avoid the simple stat comparisons. For example, if I found a chest piece, I should have to consider many things before deciding to equip it: is the level of protection higher, will the weight of the armor slow my speed, does it have any magical qualities that enhance my current skills, does it match any of my other armor (therefore bringing bonuses), at what rate does the armor deteriorate, and in general...do I look good in this??? I don't want it to take 5 minutes to compare every piece, but if they can make a quick way to compare and contrast armor as you come across it, I think this will make for a consistently interesting aspect of the adventure.