After staring at him a bit and reading other comments, I think I can put my finger on what feels off with Leo's face for me.
His "nose" or beak. With his mouth closed, it's not as pronounced as it is with others. I think they should have given some teeth similar to Donatello's, but not as opened. I was expecting Leo's mouth to be closed, actually, for a stoic look, but this doesn't strike me as too impressive.
I'm okay with the swords. They seem short, but really they shouldn't be too long if they're ninja blades. His left arm's position is definitely not as well placed as the sketch. The arm is further out on the sketch while the sculpt is more forward. Looks different, but still acceptable.
From the side, his neck does look like it's stretched forward too much, but he's meant to be facing straight on at the viewer, so okay.
I think most awkward of all are his feet. He looks like his ankles are at odd angles in order to hold onto that rooftop. Might look better in person, but the images look awkward to me.
He definitely doesn't come across as part of the set to me, I'll agree with others there.
And someone else mentioned it...his shell does look rather pristine compared to the others. Maybe that's because he spars with blades and they don't so he's taken less damage sparring with his brothers?
And Raph has a thrusting weapon, so no, less likely to cut at a shell.
I think just giving him a slightly opened mouth with teeth similar, but less visible, to Donatello would go a long way in bringing him to look like the fourth of a set. I'll hope they have a rework on it, but I suspect he'll wind up on my shelf as is.
Overall impression:
