I really like the visual style. klik described it as "fairy tales meets 70's style Holly Hobbie" -- it's all quite cute and cartoony, but there is definately a dark side to the world.
You can create your own world by placing artifacts on a map, which result in different places 'growing' -- the places interact with each other, and you can go into a place and complete a story there. Some stories span multiple places. Some characters you meet, figure in multiple stories.
Sometimes, "non-linear gameplay" means that you have to run around the world every time you have acquired a new item to see if that will open a certain part of the world that was previously inaccessible (see: Zelda, Link to the Past). I don't enjoy those kinds of games.
Legend of Mana gets it right: there is no set order to place the artifacts. There is no set order in which to complete the stories (though some stories occur before others). If you feel like adventuring, just go to some place and see what happens.
There is a ton of stuff to do outside of adventuring: growing weird fruit on a talking tree, raising monsters as pets, stuff like that. It will make your adventuring life easier, but it isn't a requirement, which is good. Some enemies are hard to beat (the ones at the end of 'Reach for the Stars' took me five tries), but they're never impossible to beat.
Overall, I am enjoying myself immensely.