Wow, this is really well done! It's polished, and works really well. I didn't complete the game, so I'm not sure how much content is really in the demo, but I've tried to write some useful feedback below.
The jump feels a little floaty, but it fits the game well. Tweaking the handling of both character and vehicles might add a to the "feel", if that makes sense. I guess I'm picturing the momentum-based old mario games - there's lots of platforming going on so it seems like a natural fit. Anyways, this isn't meant to be a criticism, I just think a little juice there would go a really long ways to maneuvering around the world.
I ended up walking off the cliff a few times until I figured out that I could summon an airship. It might be nice to have a more forgiving setting where, instead of dying on a fall, I lose a chunk of health points and get to try again. That being said, once I had it figured out, I was soon sailing around the skies.
How much content is in the game right now? Are there dungeons? Airborne enemies? A cannon on my airship would be lots of fun (nasty slimes)! I would love to play more and hope I'll have some time later to dig a little deeper.
Edit: I need to read the game pages first.