Starting in BC, Combat was always a strong choice for most powerful spec. I experimented with it in Burning Crusade, finding a "Combat Daggers" build, but I just never got into it. It always felt like it was all stuff that Subtlety and Assassination already had, without contributing anything new. On top of that, one fought with the slower weapons that plenty of other classes could use. It seemed odd to have a Rogue that didn't use daggers, as I felt they were really one of the defining aspects of the class.
But you've heard the news: Combat is on its way out, and Outlaw is replacing it.
The intention of Combat was always to be a kind of swashbuckler - an Errol Flynn type who could be either a Robin Hood-style outlaw or a pirate. But there really wasn't anything in the gameplay that reinforced that. Combat Rogues still had all of this shadowy assassin imagery that fit perfectly well for the other two specs, but in practice, Combat was the "shadowy assassin who happens to use slow, heavy weapons."
Outlaw rids us of "Sinister Strike" in favor of "Saber Slash," and leaves Eviscerate to Subtlety, replacing it with the very rapier-friendly "Run Through." Giving Outlaw "Pistol Shot" as their alternate CP-generator truly reinforces a kind of 18th century highwayman/pirate feel.
The point is, where Combat had basically no flavor to it that wasn't already part of the overall Rogue flavor, Outlaw feels very much like its own thing. A Subtlety Rogue couldn't just pick up a pair of axes or swords and replicate that feeling.
So that's the first barrier to me going down.
Then there's gameplay.
While I always liked the idea of Subtlety, the gameplay was always a little underwhelming. In Cataclysm, they decided that Sub would be the "finisher-juggling" spec, and honestly that kind of maintenance-based gameplay was never my thing (you can imagine how unenthusiastic I am about Affliction.) So though I had played with Assassination ever since Wrath introduced dual-spec, toward the end of Cataclysm my Rogue had become a pretty dedicated Assassination Rogue.
One of the big reasons was the kind of poppiness to its rhythm. Not only did Mutilate's baseline 2CP generation mean more finishing moves per generators, but you also had the passive that was initially called Waylay, allowing you to get off a free Backstab (and later Dispatch) as a random proc.
Assassination also got Cut to the Chase, which meant that Slice and Dice would stay on as long as you kept getting off Envenoms - and in Warlords they just went ahead and made SnD a passive for the spec.
But in Legion, it looks like Assassination is losing a lot of this functionality. Dispatch is going away, and while they will, like in Warlords, only have to maintain Rupture, it looks like the spec is going to be somewhat more predictable and simple.
Outlaw, on the other hand, gets a proc very similar to Waylay, which causes Saber Slash to somtimes make your next Pistol Shot free. Essentially, the poppiness of the current Assassination may find its heir in the brand-new Outlaw spec.
The lack of daggers might have been a problem before, but now that they've really established Outlaw as a flavorful variation on the class of its own, and the artifact weapons are a pair of pirate cutlasses, that objection has basically gone away.
So you might find me writing about my Outlaw Rogue once Legion comes around. I only hope that they start designing some Rogue tiers to incorporate that pirate motif. Actually, getting a tricorne for a helmet like in Bloodborne would be absolutely fantastic (give it a high, face-hiding collar and it would even work for the other Rogue specs. Make it happen, Blizzard!)