Bored.

Assassination bores the shit out of me.

There is one debuff you have to actually work to keep up: Rupture.  Other than that it's just Mutilate, Dispatch, Envenom in some order and try not to fall asleep while you "pool" energy.  I say "pool" because I think it's just a fancy way of saying "wait around and do nothing".  Assassination also has one, ONE!, cooldown (cooldown) which is unique to the spec, and it is possibly the most unimaginative and boring ability in the history of video games.  It changes nothing about your rotation, you just hit it and continue on as if nothing happened.

So, why do I care? Why not just play one of the other specs?

Because I care about killing bosses.  Assassination is about 10k DPS ahead of Combat and about 5k ahead of Subtlety in single target encounters for my current gear.  This makes it the default spec for most boss fights, unless of course there is a cleave involved.

Oh the curse of the cleave.

Hooray!  This fight has a cleave!  I can play Combat! This all sounds great, except that with Blade Flurry on you have even less energy than when playing Assassination.  Since Rupture isn't transferred to the second target you also drop it from the rotation. In the one case where another spec is viable that spec is rendered boring by the very ability which makes it viable.

I wish I could laugh at the irony, but I fell asleep between Mutilates.