Sniping depending on the Halo game took skill. Halo 3 for instance required you to lead shots once you hit longer ranges, and the aim assist was SIGNIFICANTLY smaller than what you'd find in a CoD game. As well the required headshot for a one hit kill made it a tricky weapon to actually get good with.
Though in Halo 4, Snipers have more aim assist than they've ever had, your reticle is huge, and bullet magnetism makes it almost impossible not to headshot, even while no-scoping.
IMO, Destiny looks like a Halo map-size game, meaning aside from projectiles, sniper skill should be about precision aiming with a small reticle, and very little aim assist (if any at all), and headshots should certainly be crucial. I don't think bullet drop or projectile speed should really be much of a factor considering these are hyper-futuristic weapons we are talking about, and they aught to be capable of firing much farther, and with much faster moving projectiles than modern technology.
One interesting factor that could come into play is "focus" instead of having Quick Scoping like you find in CoD, Destiny's Sniper should require you to wait a second after scoping while your reticle "deblooms" to a precise shot (effectively meaning your character has to focus down the sight before you're guaranteed a hit). Just to promote the sniper as more of a long range, patience and skill oriented dedicated weapon, and promote using your Utility Rifle, Heavy Weapon, or a completely different Special if you're opting for close quarters fights.
Unscoped, the Sniper should be damn near impossible, but still master-able with tons of practice, basically by completely removing the reticle from the screen, but still allowing you to perform a skilled no-scope if you land a zero aim-assist shot in the exact point on the screen the reticle would otherwise be.