I just played through the Yoshi demo and I have to say...
If you haven't given it a shot, PLAY IT RIGHT NOW!!! Needless to say I love it, and considering this is the first stage and there's ALREADY some aspects of it that put it above many other Yoshi games is remarkable. I wasn't expecting several mission systems, even within the normal playthrough of the level. My only gripe with this demo is that there was only one level. Like, you couldn't squeeze in one or two extra in there Nintendo? Well, I guess it's a good thing as it means people who weren't all that interested before but wants to see what it's like doesn't have to invest a lot of time at least.
So yeah, it's great and you should play it if you haven't
-Zinith's official approval