I hated this spec when I tried it back in Wrath. Compared to Destruction, it felt slow, uninteractive and overly-focussed on maintaing a full list of dots. So for a long time I assumed that it would be the same in Cata, up until I actually tried it.
Hand of Gul'dan is the exciting new spell, and forms the centre of the rotation, being your highest damage nuke, increasing the damage of your demon, refreshing the duration of Immolate, and [if talented] an AoE snare.
The spec focusses mainly on increasing the damage your minion does through applying dots and Curse of Gul'dan, but the special thing now is the benefit of being in melee range to use Shadowflame and Immolation Aura. The spec actually reminds me an awful lot of playing an Enhancement Shaman, with the hybrid-yness of being a caster in melee, using a smattering of abilities that all deal damage in slighty different ways, and that it's less about the direct damage individual spells do, but about the combinations of spells and abilities that feed into each other to contribute to overall damage.
Also, you get to transform into a huge motherfucking demon.
I just don't like refreshing dots. It's not compelling gameplay to me. I feel less like I'm directly damaging the boss than on a production line responsible for the upkeep of damage spells. I liked Destruction in Wrath basically because the weakness of DoTs at that time meant you only really had to worry about Immolate and your direct damage spells. Demonology, as I mentioned above, was the opposite.
Destruction in Cata has not only had the full compliment of Warlock dots made worth casting again, but now has an additional short duration buff to keep up with Improved Soul Fire. Demonology, with Hand of Gul'dan to automatically refresh Immolate, only regularly needs to keep an eye on Corruption. Even I am happy to deal with that.