They did this to differentiate the character from Darth Vader. Kylo Ren is a conflicted, insecure, more human character than Darth Vader ("he's more machine now than man") and that's why they made him take his mask off, to humanize him. He's not the same type of character and that is refreshing because a lot of the movie is already kind of too similar to A New Hope. I do think they should have given the character a beard though because I'm starting to agree he does look like a teenager.
I just watched the movie again and I like it just as much, maybe a bit more. I would rank it third behind Empire Strikes Back and A New Hope. I'd say it trumps Return of the Jedi mostly because the Ewoks kind of brought that movie down a bit, and the prequels...well, they're the prequels. I do think there were unnecessary action sequences that should have been cut or trimmed down in favor of more character development. As I said before the whole squid monster scene was completely pointless. Oh, also when Rey and Finn fly away in the Falcon for the first time, that should have been trimmed down a lot. I would have liked to see R2D2 and C3PO more, they have such small rolls.