It certainly helps that they're working with the series with one of the largest disparities between awesomeness of the idea and crappiness of the implementation. Even as a kid i could tell that there were problems with the plot of Thundercats, both the overall arc and the quality of the individual episodes. And even as a kid i didn't like Snarf. Obviously looking back on it after "growing up" only makes those flaws stand out even more. (Especially Snarf.)
But humanoid cats with swords and magic powers kicking ass? Basic idea = awesome!
So the new series took that basic idea, and _most_ of the character traits from the original, and redid all the crappy parts. Of course it's hard to say for sure after just the two episode intro, but the plot seems leaps and bounds beyond the old cartoon, and they even manage to skip _most_ badly done cartoon stereotypes. In fact maybe i'll change my mind after a second viewing, but even the cases where they did go with the stereotypes they handled them pretty well. Also, Snarf. He (she?) doesn't talk any more! He(?)'s just a cute little cat that makes cute little cat noises!
Oh yes, and if you still haven't watched the trailer, the animation is awesome. The only thing that's a little weird is Lion-O's new look, but i mostly got over that pretty quickly.
So yeah, i'm pretty darn happy with this. Let's just hope they can keep up the initial promise for awhile longer (and that the show will last long enough for them to demonstrate that.)