Reading through some of the one star reviews tells me that, A. a lot of those people didn't watch more than the trailers and maybe the first episode IF they've seen anything at all (which sounds unlikely from reading reviews), and B. many of them are stuck on the original and are complaining that this is not that. Which it isn't, and which it has never tried to be.
Thundercats Roar first got my attention with its animation. The animation is fun, bouncy, and silly. It's also VERY good. As an animation major who was taught under people coming from most major animation companies (Disney, Dreamworks, Laika, etc), I can truly appreciate this show's art and animation style for what it is. It's simple yet extremely detailed, and I can see and understand so many of the show's rules, even if I've never seen this show's Animation Bible (which I do really want to see, I bet it's full of neat stuff!)
I watched the first two episodes and got hooked. The colors are the perfect mix of high saturation and pastel, the style is cute and charming, the writing is really funny and the sound effects are hilarious! A lot of people seem to compare this to TTG but listen; none of the episodes of TTG that I've had the misfortune of watching have had me consistently, genuinely laughing and having a great time like Thundercats Roar. Not to mention the terrible Flash animation of TTG can't even hold a candle to the clearly hand drawn, phenomenal animation of Roar. There have been several scenes already that have simply took my breath away! The new intro alone is a testament to the amazing skill and talent of the show's animators.
Please, give this show an honest try. Watch the first six episodes. You might be surprised!