The demo's not terribly long, but it shows off enough to give you an idea of what to expect. You get to use the Light and Dark beams, some fighting with Space Pirates, a brief glimpse of Dark Samus, a trip to the Dark world, and a skirmish with the Ing.
I'm assuming you don't start out with the other beams in the actual game, though.
The new scan visor makes the old one look like crap. It's sort of like the targeting computer in Rogue Leader, where the entire object is shaded a certain color. It makes finding scannable objects a million times easier.
I noticed that the rooms in the Dark world have different names than their Light world counterparts, so it looks like Retro isn't about to get lazy on us.
And Space Pirates dislike theft, apparantly.