Kylo may not have been frightening in the way of Vader or Maul, but there was still something in his rage and unpredictability that I thought made for an effective and refreshingly different kind of SW villain. As you never quite knew how he was going to respond in any situation, or when he might angrily lash out and slice someone's head off or impale them through the heart.
How some can only see some "whiny emo kid" like Anakin I'll never understand, because to me Driver's performance made him seem a lot more interesting and complex than that. And the fact Driver actually knows how to act helps quite a bit with that as well.