It really comes down to one thing: Balance. A good FPS will have balanced weapon damage with enemy health to ensure that the player has a challenging but satisfying time, staying away from it being too easy or frustrating to the point where he's cheezing the AI to get through. But once you introduce weapon drops that dictate your primary weapon damage and armor that impacts your survivability, that balance goes out the window. And once that balance is gone, it means you no longer hit that sweet spot between too easy and too hard, resulting in a whole lot of non-ideal and (imo) unfun gameplay experiences.
This is where Destiny breaks for me. I've been playing the Alpha for a few hours and while the game is certainly neat and has a lot of cool ideas, that fundamental lack of balance has destroyed any hope of me wanting to play the game again.
In the Alpha there's a co-op mission called The Devil's Lair which is essentially a boss raid. You go in a dungeon and fight a couple of waves of enemies and it all culminates in a big boss fight. First time I ran that mission with a Fireteam (party) of 3, we literally spent 2.5 hours trying to complete a mission that shouldn't take more than 25 mins. Even though we were only half a level under the suggested level, the gap is strength was so large that it would take all 3 of us to focus fire the shielded enemies, dumping 3-4 clips each and maybe a grenade or two to bring down just one of them. Right before the boss you fight a spider tank thing and he probably had something like 5K HP and my shots were doing 7 damage each. This was so profoundly unfun and frustrating that when we were all one hit KO'd by the boss, we just gave up. Then after leveling once and buying stronger riffles we ran that mission again and cleaned house. I think we finished it in 15 mins with time to spare. I do want to mention that the game attempts to address this because the second time we ran the mission the enemies were actually 2 levels higher than before but it was still such a cakewalk it didn't matter.
In Bungie's previous game, Halo, this would have never been an issue since the weapon damage and player health were locked-in and the engagements were precisely tuned to take advantage of that fact. While weapon loot has it's appeal, I find that, particularly in FPS games like Destiny and Borderlands, it detracts more than it adds to the experience. Not to say that it doesn't have it's place, but it's not what I want out of my shooters.