Oh I LOVE this question. been meaning to post about it, but I'm glad you beat my lazy ass to it.
I actually got a list, but to be fair to everyone, and to not pick photo albums too (cause that would definitely make the biggest difference) I'm gonna have to go with... the built-in music features probably?
but what truly lacks is the essence of myspace, and the popularity of it as the leading social network. I wish this website had more of a buzz in general, but that's not a really a feature. the userbase could change a little too, there's quite a limited audience here