Bungie has a very different approach to loot than a lot of other loot games. 2 months in I hope this is becoming obvious.
For example, Diablo 3 has tons and tons of legendaries, but most are just glorified rares. There are only a few that are worth creating a build around. I sharded almost every legendary I got. Destiny has focused on making sure every exotic is unique and through (promised) buffs/balances are trying to make each one viable. No other loot game does this.
Most loot games, you're looking for that one piece of gear. Thunderfury for your Monk. Wand of Woh for your Wizard. Destiny isn't about optimal, it's about choices. That's why you're limited to one exotic. They're center-pieces that you build around. I have 4 titan exotics. Alpha Lupi is great for Nightfalls, Armamentarium for wreckin' shit, Inner Fire for Striker fun, Saint-14 for blinding fools. I want Skull Fort and No Back Up Plans. Each one lets you find another playstyle.
Bungie's openly stated they want us to collect them all. And here's the other advantage to having a smaller table. It let's you learn to synergize with and counter other players. Most loot games don't have PvP. There are 6 armors for each class I think. I can recognize each and know what they're doing. A smaller loot table maintains predicatibility that PvP requires.
Diablo 3 and most loot games are about BiS, Best in Slot. Destiny is about variety.
-----
Finally, as an aside, I fully approve of the "one exotic at a time" design choice. It keeps people who play 200 hours a week on the same playing field as those who only play 10. After all, they each can only have one exotic equipped. The hardcore player doesn't get a power advantage like most games, they get a choice advantage. They can run any of 12 different styles, but they're all equal to the guy who only has one exotic. It was a brilliant decision.
Ok, but these design choices are built on the pretence that the underlying gameplay loop is compelling enough and the content (worlds, enemies, objectives, etc) are varied enough to ensure the player is having fun between the much longer gaps between the gratification of high-quality loot drops.
It isn't right now.
The feel of the movement and the shooting is fantastic. First class. Exactly what you would expect from the guys behind Halo. BUT:
The enemy AI is pretty much non-existent until you hit lvl 24 missions, and even then, only 2 of the 4 races demonstrate any kind of intelligence as far as flanking, or working together, or retreating and re-engaging. The Hive and the Vex are basically just reskinned versions of The Flood.
Coordination does become critical on higher difficulty settings, but even then, it's purely about calling out targets, reviving each other and focus firing. The classes are basically irrelevant by the higher levels outside of specials, and none of the missions in the game actually require players to coordinate their different talents. You can run any combination of the 3 classes without issue.
The bosses are boring chores to fight against. They're generally immobile, have one attack apiece, and rely purely on being a bullet-sponge and surrounded by ads to pose any challenge.
The scenarios in the missions are simplistic and repetitive. The Raid adds some extra elements, which is nice, but in the case of those boss battles, glitches and shoddy execution kneecap otherwise conceptually interesting puzzle-boss fights.
You can go on and on with this. The point is, if you're going to design a loot shooter with a more limited loot table in mind - even for great, intelligent reasons such as the ones you mentioned - you damn sure better make sure the 'shooter' part is satisfying enough to provide gratification outside of the loot drops. Right now, Destiny doesn't really. The competition with friends to try and get the best gear, or the simple 'collect them all' impulse is really the only thing pushing a lot of people forward right now.