The game is at its core well designed. The problem is in the details and the execution. It fails super hard here. A game that's oriented around loot and a constant "carrot hanging in front of you" progression requires that progression consistently exist and the avenue for progression to exist and be meaningful.
Let's break this down a bit. The game is a loot game. Let's not delude ourselves in that front. In order for a loot game to be satisfying is broken out into a few quintessential pillars: diversity in items, diversity in builds, and method of getting said items. Going into the first portion, there needs to be a diversity in items. I don't mean this just in a vaccuum of what different items are available in the pool overall but what modifiers that distinguish on item from another. Furthermore, there needs to be a synergy with a subset of items. Right now this also doesn't exist either which is a major problem We have visually distinguishing sets but the sets themselves aren't so different from each other to make a difference. Furthermore, the pieces in the set don't synergize with itself to make a difference either. Fortunately, the items right now in Destiny are distinguished from each other in the weapons section. They are not in the armor section which is a major problem. I'm going to use Diablo as an example but there simply isn't enough meaningful armor modifier diversity in exotics to create distinguished builds from each other. In Diablo there are multiple ways to play the same classes and this is done by item sets and the skillsets. This sort of exists in the skill side of things in Destiny but not in the items. The Mod slots sorta feed into this but it's not meaningful at all. It's not enough. As it stands now, Exotics actually be the legendaries and there needs to be an even more meaningful tier. The game is not in a good spot and I was hoping that D2 would've addressed this.
Let's move onto the last pillar, the method of getting items. The problem with Destiny isn't the amount of content. There's way more to do in this game than the first one and there's no denying that. Unfortunately the content is made obsolete and that's due in part to two things: the soft Power Level Cap on items you can get a week, not being able to increase power level outside of a small subset of activities. Let's talk about the soft power level cap. Currently the cap is 350, which is very far out. Not that big of a problem except for the unfortunate fact that you can't continually increase the cap throughout the week. Once you complete the activities that give you stronger loot, you're done. In this situation, the milestones. This feeds into the second point: small subset of activities. When you can only get meaningful item from a few content in the game (nightfall, raid, etc) then you have a serious problem. You created all this meaningful content like Adventures, Patrols, Public Events, and Lost Sector but no one is actively doing them. Why? Cuz the rewards are garbage. They don't feed into progression. The playable content is only as good as the rewards they offer.
The entertainment of loot games is to get stronger yes. Unfortunately Destiny focuses so much on seeing your Power Level get higher that they don't think about the actual end game in loot games. Reaching a power crap relatively quickly so you can try out different items and builds. Destiny doesn't do this and this is one of the overarching themes on why it fails.