Comment by sublinear

Many (most?) things are rendered without image files.

I think it's the overall flattened look, use of overly simplistic gradients, bland fonts, animations that feel dead, etc. The lack of details all adds up to feeling cheap. It's not any one of them in particular, nor something I'd be able to advise on. That needs a human designer. It doesn't take much sophistication to notice it, though.