Other than its kind of self explanatory looks, I think what really makes a teen skin is that the skin is already restricted to fit around one theme. That theme being a teen. Wether you can bend that to also fit into different type of categories like aliens or whatever comes to mind still doesnt change that it is already lacking in originality and creativity because of having to make it appear more human than what it really could be / being limited to one view. It's also a very common type of skin that appeals to a huge portion of the minecraft community, which is why I believe it doesn't hold as much value as other type of skins. They're also all very similar because they are so massively and frequently made. They're obviously not all exactly the same, but its rare to see anything that has taken several weeks or maybe months to be made into something more polished and different than the norm since they all roughly follow a traditional basis/format.
It just kind of gets boring to look at more of the same, especially since theres a lot of room for improvement too.
Just some thoughts though..
