Diggin' the smoke effects and hi-def hair. It really captures how truly evil this character could have been, despite the lack of dialogue, or real interaction between the main characters and himself beyond a few words and some growling.
No, he just didn't get enough development. That's all. He has a lot of potential to be this dark and malevolent constant; instead he suffers from what many other characters suffer from: the plot device.
Honestly, I really liked him. He looks like a pony version of Makuta, but lacks sufficient development. I wish we could have kept him longer.... But remember: when he was defeated, his horn survived.
Please tell me I'm not the only one who likes him BECAUSE we know so little about him. He'd fit right in with those 80s MLP villains; unclear motivations and backround, yet insane and almost primal motivation. I'd dare say that in Sombra's case, less is definitely more, and that ranks him up as one of the creepiest MLP villains by far.