I much prefer an understated villain like Nero. When he flipped and went homicidal maniac on Robau, it was much more chilling and effective than if he had been screaming and flexing his strength from the first moment.
Do we truly understand his motivation - not really. But look at the headlines every day - do we truly understand what motivates those who decide to shoot an Amish school full of children or anyone who walks by on a college campus, or a serial bomber who sends mail bombs for decades, or terrorists who fly planes into buildings. Surely they are not wreaking their vengeance on the original source of their pain, any more than Nero can logically blame anyone for the death of his family or his planet. But his pain, and his psyche, demands payment, therefore he chooses the best candidate in his eyes - Spock. In the first days after my son's death, I entertained wishes of tearing his girlfriend to shreds. Although she didn't kill him, she had culpability, and I wanted revenge - grief does that to you, you know - you don't think straight every moment. Of course, I never even came close to even being unkind to her, let alone hurting her in any way, but I certainly get that part of Nero's story. And it is not unusual for such pain to fester for years and get stronger and more unreasonable, as happened to Nero. And we are talking about a Romulan here, after all.
We like all our dots connected in stories and want all the facts out there. But sometimes characters are more interesting if it's not all there and you have to think about what they're doing and thinking and what's motivating them. I have read enough times this week that Star Trek should make you think - well here's your chance. Think like a profiler (they many times have no info about a criminal) and get into Nero's psyche.
I totally agree that Nero's friendliness and ordinariness was totally awesome - "Hi, Christopher. I'm Nero."