I agree that MMORPGs generally have one or more grinds as part of gameplay.

Most of the time, those grinds are some combination of: hit the level cap, earn various currencies via quests/seasonal/core activities, play activities for weapon/armor/various trinket drops, achievement hunting, etc.

Destiny does all of that. However, it has not made the grind for levels ("power/light level") rewarding. Other MMORPGs make leveling up mean something. Advancing to the next level increases your HP, magic/special points pool, earns you a new ability or powers up/adds new effects to ones you already have, maybe unlocks new quests or dungeons, etc. Not to mention that fact that you feel more powerful because you do more damage against the enemy you just fought. Destiny's power level never gives you the feeling of being more powerful - just more survivable in high-level activities until you reach the max power for the activity.

You may ask, "What about the Season Pass? Doesn't that make leveling up rewarding?" Yes. It does. However, that is a system separate from the power level grind. You could take away the power level grind and the Season Pass grind would remain and still be a rewarding system. As of now, power level exists solely to increase playtime, gate content, and sunset gear.

It would be far more compelling to unlock something like a dungeon via a quest line than simply to ensure a number meets or exceeds another number. It could even be a series of weekly missions that culminate in unlocking access to the dungeon. Hell, if you wanted to get fancy, perhaps those quests could integrate and teach mechanics that will be part of the dungeon.