A little of both, I guess. Mostly attitude/assertiveness. If you watch Wes J, and compare what he "could" do to what he actually does, it's that. Whatever Kobe, Tim Duncan, and MJ had/have. The "stop screwing around, we need a bucket here and it IS going to happen" feeling. Not only the ability to have that feeling, but the knowledge and maturity of when to use it.
Rubio is a mature player in some ways and he knows to kick that ability in at the end of games, but nobody, including him, recognizes the need to kick that same urgency in at the beginning of games so there isn't a hole to dig out of. You can talk about it all you want after games, but nobody plays with urgency in the early minutes. Sloppy.
I guess that's why I call it a maturity issue. In our case, not only do we not have the attitude but we don't have someone with the ability to score regularly without disrupting the flow.