Well, it depends on the way it is implemented. I, for one, would have no problems if ADS was implemented like in Bioshock, where it is completely optional to use it (it only gives you a little bit more zoom, and reduces your aiming sensitivity, but doesn't change the level of accuracy of the weapons)
But there's another, way more annoying kind of ADS aiming, the kind that use most shooters nowadays. It basically penalizes your accuracy when not using ADS, forcing you to use it most of the time, wich totally sucks. Not to talk about the fact that going into ADS slows down your character, which sucks even more. If you also add the accuracy penalties for other actions, like jumping, then you can have a pretty good idea why people would hate ADS.
There's also the fact that Bungie was one of the very few developers that still made FPS's without ADS (a dying breed, sadly) which, in this case, makes it even more worse.
You could say ADS adds realism, but in the case of Destiny, a sci-fi / fantasy game where things like "space magic" exist, it wouldn't make much sense. And there's also the fact that there are much better ways to introduce the same kind of gameplay restrictions in way more interesting ways. See CS:GO, for example; while today's ADS shooters only require you to press LT, Counter-Strike actually makes you slow down your character and crouch to be accurate, and all that with the added benefit of not having to have half the screen obscured by your own weapon.