IGN describes the new version as follows:
You have the ability to toggle back and forth between how the game originally looked, and how it looks now, and holy moly the difference is night and day. Thanks to the new lighting, shaders, and other technical enhancements that flew way over my head, Manny and company now look incredible. Their in-game models appear to be nearly identical to their cut-scene counterparts, which is great. The game is still presented in 4:3, but you have the ability to stretch it to widescreen (for the love of everything good, please don’t do this). The borders are black right now, but I was told that the developers are toying with some ideas for what could stand on the two sides of the screen.
In addition, Double Fine has also added some bonus features to the game:
The final big change I stumbled across was the awesome inclusion of nodes scattered throughout the world that contain small snippets of commentary from Tim Schafer, Peter Chan, Peter McConnell, and a ton of other folks who helped make the game so special back in 1998.
Polygon has also shared their thoughts on the demo, and have confirmed that the original's tank controls will be an option for the purists who prefer to play the game that way.
It looks like Grim Fandango Remastered is shaping up to be great. The rest of us will get a chance to play it for ourselves once it's released in early 2015.