might want to make the footprints in the sand a dark brown, though thats a minor issue I think- the style loks neat so far, I think a little mock-up-scene might give a better idea of the look- in particular i want to see how the outline of the character works compared to the scene elements which don't have an outline.
Also I think the profile of the character needs a little touching up- his nose looks bigger than in the front view, and the perspective of the goggles seems a little funny to me. Maybe get a little curve in his spine too?

It might help to do a little mock-up to show the potential of the tiles, right now its hard to say- maybe throw in some flowers/ shrubs to make it a little more interesting?
Also there isn't a lot of interesting texture ot the grass or dirt, maybe make and actual shape that mimics the border of the grass or something for one tile, and leave the other a solid colour?

looks fantastic, great preview on the video. The balloon drop into the level reminds me of Kickle Cubicle for NES which was a sweet game.

I think It has a really nice style. I don't think the composition is that big of an issue, it can shift around in a game depending on how far the camera goes near the borders of the level as well. Right now it seems like you guy is about as high as he can go, which kinda works for the scene.

I'd juts put in some more immediate background treetops etc near the bottom of the screen and behind the big trunk, just so it breaks up the blue a bit more, and makes it seem more like something is down there.
Making the ground plain not so flat might also give it a better feel. In the picture posted by morden the ground is just a bit bumpir than yours so it feels really organic.

Also, if you do have a character in the final version, (though you may have thought if this already) standing on the right platform standing/leaning against the tree trunk would look cool.

I think the colours and style are a really nice start!