I think it might be worthwhile to up the contrast of the two greens in the grass. It's very hard to distinguish between the colours unless I'm looking veeeeery closely.

That's not bad, but it's still not what I'm looking for. The reflections on the diagonal bits still don't resemble the above grass at all. Thanks though.

You really don't want your terrain to take more attention than your sprite(typically).
Thin may be good for this case.  Your terrain is already 60% of the mockup and the other terrain in the background fills almost the rest.  If you go much "thicker", your terrain may pull too much away from the action.

Are you running off of a 2x zoom or anything?  That might help a lot to bring the attention squarely onto the subject and it will also make the tiles feel much more fleshy up close.

Thanks for posting at 1:1 scale 

In that case, he looks fine and should match other sprites you add in.
Since he is really pushing the boundary, he might bleed up 1 pixel when you make him run but that might look acceptable.
Just a suggestion though that it can be interesting for gameplay (as well as visually) if the hero is say 1.5 tiles high, and needs to crouch to fit through smaller gaps.
You can afford it too with that resolution.

I also feel the sprite's colours are a little subdued and could do with a touch more saturation and/or you could further tweak the colours of the surroundings.
Notice for example, coffee's version has a brighter and more saturated cluster for the belt buckle?
This helps give him visual priority.


The thick white line in your tileset is a bit overpowering and taking away from the all-important border on the edge of the playable space, so I had 5 minute fiddle to blend it and make some sharper, more planar edges...