Not just bad interface design: Incomprehensible interface design. Interfaces which have nothing in common with any other interface the user has seen before.
With HTML you at least have a stronger pressure to use the same text boxes everyone else does, for example: There's more standardization of basic building blocks. You can go whole-hog with reinventing the square wheel only this time making it diamond-shaped and made out of cream cheese, but it's harder.
In Flash, apparently, there's less of a counterbalance to going off entirely on your own and making something only you can understand.
How is bad interface design a problem specific to Flash? Surely you can create a bad interface with modern tools just as easily?