The huge difference is that Ghorn's wolfpack rounds had tracking, whereas Y1 Dragon's Breath only left damage in a very specific area that--as Korny said--usually no longer contained an enemy.

The way I see it, Dragon's Breath (both past and present versions) is an interesting weapon that has some very specific uses, but is ultimately undercut by Bungie's reliance on bullet-sponge enemies. For example, 1 or 2 Dragon's Breaths in the right hands can make a huge difference during the Abyss in Crota's End. They're a great way to clear "safe zones" at or between the lamps, giving your team some room to breath. Problem is the Crota's End raid is filled with Major-class foes, making Gjallarhorn the clear go-to exotic for most people, with the possible exception of Ice Breaker. Back when we were all level 30, CE Hard Mode was challenging enough that most teams needed all the DPS they could get, and that just didn't leave room for Dragon's Breath or most of the other exotics.

Things are better since Taken King, although if I'm going to take an exotic heavy into any end-game activities I find myself using Sleeper or Raze-Lighter almost exclusively. Although I do want to try taking Dragon's Breath into the Oryx fight one of these days. I was actually just talking about how the Oryx battle seems to be one of the most interesting encounters in Destiny in terms of the way many different weapons can fill niche roles within a team. Every time I run it, I see a mix of Sleepers, Black Hammers, Touch of Malice, Eirene RR4's and 1KYSs, Quillem's Terminus, etc. It's not like the Crota days where we reach the boss and 6 Gjallarhorns come out :)

Gjallarhorn was/is great against everything. Groups of enemies, Majors, bosses, Guardians. Not only did you have a fast-flying rocket launcher with tracking and Horseshoes, but after the initial blast, you'd have a whole slew of mini-rockets hunting down/stacking damage on enemies. There was a time where Gjallarhorn saw more use than most other weapons in a single mission/match.

Well, never saw much use out of it with mobs. Saw a lot of wolfpacks seeking already-dead enemies to make them that useful.

Nope, Napalm covers a far greater area, lasts three times as long, and can be used over cover. Pair it with a Defender or Skyburner's Annex, and you're spewing orbs left and right.

Well, that could've been implemented with the flares, I think, and be a little better. I don't really like how the napalm is confined to the ground, though I do appreciate the extra seconds it stays there.