1. He Can Be A Bit On The Nose
This criticism aims at JGL's dialogue writing specifically. He does a lot of great work with his dialogue, but every now and then he lapses into transparency. All of the themes and messages Don Jon should subtly imply are usually spoken aloud, taking any ambiguity and mystery out of the experience. This can be chalked up to the characters he was working with. As someone from an Italian family, I can tell you that we say what we mean and don't tend to get cryptic. Backhanded, yes. Subtle, no. But at some point you need to say "to Hell with realism" and remember that movies are better when they're less obvious.
I actually really enjoyed "Don Jon", and recommend it to everyone mature enough to handle the sexuality. It's not perfect, but it's a seriously promising start for JGL as a filmmaker. What do you think?