FWIW I have NO PROBLEM with Adam Driver's casting and the character.
People need to stop throwing around the "Emo" adjective.
For me, part of it was the presentation. He was a cool evil guy with a temper & a scary voice...until he took off his helmet in front of Rey. I think that is what ruined his credibility as an evil-doer. They should have saved the de-masking until he met with Han on the catwalk, because then it would have highlighted that he's somebody's son, not just a Bad Scary Dude with no past & a scar-free face.
Second, they wanted to have Han's death at Kylo's hands be his turning point to cement him into the dark side. The effect didn't work, because I didn't think "wow what a bad guy!", I thought "wait, really? The director killed off Han? Dang, bummer ". So the intended effect wasn't achieved for me. I mentioned in an earlier post, I think it would have been MUCH stronger for him to have faced both of his parents, tried to kill Han, but Leia jumped in front and got stabbed instead. Then, Leia having died in Han's arms, Kylo decides well this is it for me, this is my chance to go full Vader, and lops off Han's head, ending with their bodies falling into the abyss and the audience going HOLY CRAP!
But instead, in the interrogation scene, my first thought was "Josh Groban is the bad guy?!" and then "he should be in an Herbal Essence commercial!" Followed by Han's death, where I was more sad & surprised that they killed off his character instead of shocked that Kylo did it & now that makes him a Real Bad Dude. So my knee-jerk reaction is that he was poorly developed as a bad guy because they could have made it soooooooo much more powerful and really established a true villain for the series. But, I also understand that SW is now owned by Disney & they want younger audiences to see it, so they couldn't do something like that. I also understand how they're trying to develop him textbook-wise, but again, the effect didn't work for me. It's really hard not to see him as an emo teenager who throws temper tantrums.
And I think the reveal of the de-masking is what worked for Vader. The whole time, he was in black, face veiled, low voice, doing bad things, and the
one glimpse you got of his head was this freaky part in Empire:
It isn't until he defeats the Emperor & has his helmet removed that you see he is a broken, scarred man who now wants to atone for his sins. If they had shown that pale white face at the beginning of the movie, it would have been a joke! So I think that's one of the weak points of the new film...they did need to reveal Kylo's true face, but it could have been done a thousand percent better with a far stronger effect for the audience to mentally put him into "bad guy" territory.