The basics for uniqueness in our specs are already present. They're just not utilized well enough, that is the problem. All our specs have been made sustained dps specs, which is why we all feel they play too similarly. Combat and Sub are indeed a little hard to distinguish between. However, that doesn't mean all 3 specs are completely identical either.
Combat is heavy on auto attacks and the bulk of its damage comes from its Adrenalin Rush. The play-style has you dishing our as many Eviscerates as possible, in order to the reset the cooldown on your Rush. The entire spec is, in essence, built around using your cooldowns, refreshing them as quickly as possible, and repeating the process. Add to this one of the games (if not THE) best cleaves and you have a unique spec.
Assassination is very different in comparison. The abilities cost a lot more energy and the energy regeneration is much lower, giving the spec a distinct feeling. Pooling is actually advised, a feature neither Combat nor Sub make use of. Cooldowns aren't that big a factor here, however, like they are with Combat. It's much slower and heavier. Here's the problem. Poisons are applied via auto attacks, and having your damage come from auto attacks gives the spec more or less the same feeling as Combat, only not as fast paced.
Sub, I will say, is a very problematic spec. It needs a rather serious overhaul, or at least some serious changes. The general rotation is EXACTLY the same rotation as Combat, except with a Sinister Strike that only works from the back. The spec still has bonuses that ooze the concept of "mandatory pvp spec" which all need to go. And the spec actually needs to find purpose once more. Pre 5.0, this was our burst spec. Now, the spec just doesn't really make any sense to have.
Add to this that the abilities used are also the same around the board, meaning there's very little differentiation between the actual commands given to your character, and you have yourself a problem of homogenization.
There are clear differences between the specs. There are clear strengths to each spec. But they have just been utilized SOOOOO poorly and there's been done so little to the class that everything just feels kinda blended together and indistinguishable from the rest.