For me, I think that they lost the element of surprise and wonder that came with new dragons. So instead a nice, rare dragon coming in for an episode; there was a new dragon every episode that it became commonplace... and consequently, not as fun.
RTTE definitely had its fun moments, THE CHICKEN!!! But it got tiring after the first season and a half. It didn't look like they were putting much work into the dragons' designs either... O_O