Utility vs DPS:
As Ghostcrawler mentioned back in the Rogue Q&A, Subtlety is a utility tree. Many of the talents are interesting buffs to our crowd control, energy management, stealth, or entirely new functions for a rogue. With talents like Honor Among Thieves (which GC touched on specifically) and the candidate for 'best rogue talent ever', Relentless Strikes, Sub has the capability of completely changing how a rogue thinks about the basic elements of the class.
Combo Points are now gained in a steady stream, leaving you to make the interesting choice of which finishers to execute. Energy regeneration completely reverses, and it becomes a race to burn your energy as fast as possible instead of the current paradigm of pooling until it's absolutely necessary. Between Enveloping Shadows and Cheat Death, our survivability reaches an unparalleled level. A rogue's ability to survive outside of CDs is something else GC touched on. No rogue picks up a non-DPS talent if they can help it, and so, until that issue is addressed, we will continue to be "glass cannons". If you look at the Sub tree in detail, each tier provides at least one new utility talent for consideration. The problem is that all of this utility comes at a precious cost: DPS talents.
When using a Combat spec, there is typically one 'filler' point, and it usually gets thrown into Endurance for its Sprint cooldown reduction. Every other talent is selected purely from a 'maximum DPS' perspective. Mutilate isn't much better off, with a possible two points tossed into Fleet Footed or another Tier 5 talent. Sub Rogues really get to pick their talents in each tier, because often there's no "obvious DPS talent". This flexibility is what currently places Subtlety dead last in pretty much every environment.
If Subtlety were to suddenly start doing the same DPS as a Combat or Mutilate build, players would immediately flock to it. Tons of utility and dramatic play style changes without sacrificing any raid performance is enough to draw even the most skeptical to the spec. That's the reason you don't see a 'Hunger for Shadows' at the bottom of the Subtlety tree, adding 30% damage to all attacks as long as you remember to keep up the debuff. In the current design paradigm, utility and DPS are mutually exclusive. If Blizzard baked in so much DPS passively to the class that talents were irrelevant, it would cause the current DPS talents to make us dramatically overpowered. We're pretty much stuck without a complete rebuild.
Why Cataclysm could be the silver bullet:
In the migration from vanilla WoW to TBC, we saw a 30% spike in Stamina values across the board, which really redefined our preconceived ideas about how much life a player should have. In the move from TBC to WotLK, we saw entirely new stats explored, and a more diverse and powerful loot table than every before. Now that Cataclysm is on the horizon, we're going back to the basics. The massive overhaul of gear, abilities, and talent trees will be the largest undertaking Blizzard has ever attempted to pull off in WoW.
They've been building on a foundation of abilities, talents, and stats that was constructed over 5 years ago, before most of the staff had ever cut their teeth on an MMO, let alone one of WoW's size. With Cataclysm, they get the chance to rewrite all of that. I believe that the simplification of gear, the redesign of the talent trees, and the Mastery stat are going to go a long way in terms of making every spec viable in nearly every realm.
The two changes that will save Subtlety are the ideas of 'Sub Mastery' and the Mastery stat. It would allow a rogue to drop points into those talents that were previous devoid of any DPS gains and still do more damage, by adding a 'damage per point invested' effect into the tree. For example, with every point in the Subtlety tree, we could gain 1% extra damage against enemies. After 20 points invested, we start regenerating energy faster. Once we've dumped 40 points into the tree, we could see a tweak to Honor Among Thieves to give us one combo point every 5 seconds for free.
With these types of fine-tuning knobs available, it will allow Blizzard to make our specs less dependent on the 'magic talents' like Prey on the Weak and Hunger for Blood and more focused on fun talents like Cheat Death, Improved Sprint, and Vigor. Wouldn't it be so much more interesting to play a game where you choose which spec's playstyle you prefer, and then Blizzard is the one that worries about making sure that you're not holding your group back by doing so? Instead of being restricted to one single class, we would have 3 different modes of operation, each as powerful as the other two.
Of course, all of these changes (and Cataclysm itself) are still marked with the infamous 'Soon' stamp, and there's really no way of knowing how long it will take for the developers to get rogues to the place where your spec doesn't matter as much as your skill. Until then, Mutilate and Combat will remain the kings of our castle, while Sub rogues wait in the shadows for their time to shine. Or their time to be invisible, since glowing would probably be something a stealthy rogue would want to avoid.