Obviously Rogues are going through some big changes. One of their specs has even been renamed. But while Outlaw has got its pirate thing going on (a really huge improvement to the flavor of the spec, in my opinion,) I'm really excited about where Subtlety has gone.
See, my Undead Rogue was the first character I got to level 70 way back in Burning Crusade. Soloing was tougher back then (mainly we had crap gear) and so I fell in love with Rogues and particularly Subtlety's ability to vanish and hide and wait for a better opportunity to take things down.
The problem is that a lot of those stealth shenanigans didn't really work in group content. And Subtlety got a lot of PvP utility stuff that, back then, meant they had to nerf its damage significantly (I also wasn't as good of a player back then, so there's that.)
Over the years, they've tweaked Subtlety in various ways to give it a real niche in PvE. Actually in Warlords it has managed to be pretty effective, but it never really felt like it was living up to its potential. Shadow Dance is fine, but otherwise the spec didn't really play much into the "striking from the shadows" fantasy.
Subtlety now has a lot of shadow-based abilities. We have Shadow Blades as a major cooldown (I think it's basically the same as it was in Mists, but now only for Sub Rogues.) We also replace Rupture with Nightblade, a DoT/snare. Shuriken Toss and Shuriken Swarm both reinforce the Ninja theme, and have cool purple-shadowy animations.
We also have Shadowstrike now instead of Ambush, which has a built-in Shadowstep (or Cloak and Dagger.) Thankfully, we also still have Shadowstep, meaning that Subtlety can move around quite a bit.
So let's break the baseline spec down:
Shadowstrike is our opener and something we'll want to use during Shadow Dance. It hits the target for a solid chunk of damage and also puts us behind them if we're not in melee range already.
Symbols of Death is a new maintenance buff that we can only cast while Stealthed or Shadow-Dancing. It lasts a good long time, increasing the damage we do. Also, after casting it, our next Shadowstrike is a guaranteed crit. I don't think the design intent is for you to use this every Shadow Dance, but it is off the global cooldown. It costs Energy though, so I'd wait for the min-maxers to figure it all out.
Backstab is our primary CP-generator. You can now use it from any position, but it will do more damage if you're not in the front 90 degrees of your target. Luckily, you can replace this with Gloomblade on your first talent tier, which does Shadow damage and has no positioning advantages (which I think makes it the obvious choice for soloing.)
Nightblade is a finishing move that deals damage over time. It's more or less just a Shadow-damage Rupture, but while it's on the target, your melee attacks will slow the target.
Eviscerate is still the filler-finisher you'll use when Nightblade has plenty of duration left.
Your finishers will reduce the remaining cooldown on Shadow Dance by 3 seconds per combo point. Additionally, your autoattacks will also sometimes generate combo points.
Shadow Dance works pretty much the same, but now has three charges. Given the refreshing your finishers will be doing, you can work these in very frequently. This should make it fairly easy to prevent Symbols of Death from falling off. I've typically been able to get off three Shadowstrikes per Shadow Dance, and given that Symbols of Death is off the GCD, you should be able to fit that one in there as well if you have enough Energy.
For AoE, you have Shuriken Storm, which works pretty much like Fan of Knives, except that it has built into it that you generate a CP for every target hit. I think you'll probably want to multi-dot with Nightblade in typical trash packs.
Shuriken Toss replaces Throw, but generates a combo point when used. The downside is that it costs Energy. Still, a decent option for when you're on the move or just can't be near the target.
The spec is still a bit maintenance-y, but I think it's much more proactive and less focused on just making sure buffs are up.
You'll probably want WeakAuras to track Symbols of Death and Nightblade - the latter doesn't last super long. You also might want to track Shadow Dance charges - you'll definitely want to use them as much as you can, but also make sure that you can maintain Symbols. And like all Rogues, a WeakAura to track when you have full Combo Points is never a bad idea.
As a reminder, Combo Points now appear below your Energy bar, reflecting the Warlords change that CPs are "on the Rogue" now instead of the target, something that I think should have been true in Vanilla.
Also, rather than Recuperate, all Rogues now have Crimson Vial, which costs Energy, has a cooldown, and restores 30% of your max health over a few seconds. Definitely something you'll want to use frequently especially when soloing.