I've played Assassination relatively consistently for a while now. When I first started my Rogue, I sure as hell focused on Subtlety, as I loved the idea of striking from the shadows, and back then there were even talents that made stealth more effective against your enemies (there was even a piece of class gear that increased your "stealth level," something we sure haven't seen for a while.)
The thing is, Sub always struggled, both to live up to its fantasy and to pour out the numbers that the other two specs could. There was a time when you'd hear people talk about making sure there was balance between "both" Rogue specs, because any serious players would only consider the non-Subtlety ones.
This gradually started to change over time. In early Wrath, the talent Honor Among Thieves gave them the combo-point generation to maintain a lot of different effects at once, and with Mastery introduced in Cataclysm, they became the spec of finishing moves.
It's been that way for a while, and while Subtlety has finally become a perfectly viable spec - actually the top single-target spec in Warlords - the finishing-move theme never really matched the fantasy of being a ninja striking from the shadows.
There has been a bit of that theme, mind you, with a cooldown introduced in either Wrath or Cataclysm called Shadow Dance, but while this is on a short enough CD to give you a lot of Find Weakness uptime, it's not at the core of the spec that way you'd hope.
Oddly, I remember thinking years ago that Shadow Dance would feel more fun as a proc, rather than a cooldown. Lo and behold, that's exactly what it will be doing.
At the moment, the design is for the proc to literally put you in stealth, except when you are in PvP. I assume there will be some sort of design work to make sure that this doesn't make it literally impossible to use finishing moves while soloing out of fear that any proc will make the target reset (that might be what the shadow decoy is for - giving them something to attack.)
On top of this, the new Sub-only equivalent of Ambush will be Shadowstrike, which effectively mixes up Ambush with Shadowstep, or alternatively, essentially makes Cloak and Dagger baseline.
There's also a new DoT finisher to use instead of Rupture that does Shadow damage. Not very different mechanically, but certainly more on-flavor. We don't know whether Sub will still have to juggle finishers. Currently they have to maintain SnD and Rupture, filling in with Eviscerates if they can. The other specs only have one "maintenance" finisher (though Feral Druids, who are almost the fourth Rogue spec, are in the same boat as Sub, with Savage Roar and Rip to maintain.) While perhaps there are some Rogues who like having the management aspect to the spec, I really don't feel like it plays into the fantasy (if it would for anyone, I'd say Combat, but Combat is now Outlaw, and has its own new stuff going on.)
This re-orientation of the spec's major theme is something I welcome with open arms, and I find that I might actually go back to Sub as my Rogue's main spec (actually, I'm also excited to try out Outlaw.)
There is another really nice change coming for the spec, which is that you'll now be able to use Backstab from any angle - it's just that it'll do more damage (which I think we'll see balanced as its full damage) from behind (or the sides.) You still won't be doing max damage while soloing, but your signature CP-generator will finally be usable when you don't have a tank.