I noticed that there are a multitude of ways to be killed near instantly while playing both in the crucible and in many of the story missions in Destiny.

THERE ARE BOSS SPOILERS IN THIS THREAD

Story:
Every. God. Damn. Boss. Every damn boss can one hit kill (OHK) you if you come into melee range, or they fly around like Sepiks Prime and you can't touch him. I mean in Halo, if you were quick enough you could melee kill a Hunter if you wanted because their melee hits could be dodged. But here there is almost no way for a blade dancer to slice up a boss, or a titan to stomp a boss. I'd rather the boss kick us and send us flying away like Sauron or a Halo Hunter. But I do like the Devil Walker's OHK with his cannon because you have ample warning. But things like Rixsis (the giant Fallen guys) stomping and OHK'ing you is unacceptable. Phogoth's melee makes sense because you'd have to be a moron to try and get close to that menace, but even then you can easily dodge his hulk smash.

Crucible:
Shotguns, snipers, fusion rifles, missile launchers, supers, the Titan's sprinting tackle, interceptors, charged up melees...

I mean...really? Maybe we should also have equally more abilities to counter OHKs. Why can't the Defender shoot out of his bubble shield? The time to kill is already abysmally low, why add OHKs almost everywhere? In Halo, you knew when that Spartan Laser was going to send you out, the little laser designator gave ample warning of your impending doom.

The Fusion rifle OHKs at mid range, the same range as most primary weapons. I feel like it's charge rate should be slower and charging sound should be louder because we don't have much of a visual warning. Or just make it a shield draining weapon like the charged up plasma pistol. You'd then use the fusion rifle to deplete a guardian's or guardians', standing too close to each other, shield(s); then finishing them off with your primary. The beam wouldn't melt through the shield and bleed into health, it would only do shield damage from the first round, the second round would kill the target. This would turn the fusion rifle into more of an ambush based weapon, you'd get the jump on a guy, laser him,switch, head shot done. Or a teammate would laser him, and your teammate would mop him up while you laser the next target. If you run in point blank with this version of the fusion rifle you'd be punished by a shotgun but not a sniper. This would give a rock paper scissors like dynamic with the special weapon category.

Shotguns are working as they should, as well as snipers, rockets and HMGs.

Defensive Supers: I feel like Defensive abilities need to charge up faster. An offensive guardian with an offensive super would naturally get on the offense, gets kills and gets his super to kill more. Whereas the dynamic changes for a guardian focusing on the the defense; he needs to go on the offense to get his super, and when shit hits the fan he hides in his bubble...really bad chain of events there. Why is this bad? The time to kill is low, you have no time to react to most OHKs, will you really drop your bubble in time or will you be killed in the process? Now if you give the Titan the ability to shoot out of it, it changes the "hiding in his bubble from ambush waiting for his buddies to show up" to "super charges himself and fires from cover eliminating the ambush". Before you scream "THAT'S TOO OP!", choosing the perk to shoot out of the bubble would decrease the time the bubble lasts on the field, problem solved.