I recently purchased (pre-ordered technically) Starbound, which is a side-scrolling platformer, with exploration and building elements. The shortened version of the story is that you are last of a crew sent out to explore strange new worlds, to boldly go where no plant-thing or fish-thing has ever gone before!
It has that strangely addictive quality found in Minecraft, though combat is a bit clunky, as you aim using the mouse, but move using keyboard controls (standard WASD).
I've mostly confined my building to my ship, where I've set up a small farm and crafting stations. Some of the randomly generated mobs are rough, and some of the environments are as well. Since each world is random, it does offer a good deal of exploration to do, but so far I've only found one inhabitant of a world, whom I promptly slaughtered.
There are mods available, and probably more of a story arc to follow once it gets further in development. It is in beta, so not everything is complete.
All in all, well worth the $15 IMO. I enjoy both building sandbox games, as well as platformers (Mega Man, Sonic the Hedgehog).