It's mostly the artstyle, some may not see a difference in the character design, but I see a drastic difference. "WhAtS tHe DiFfErEnCe?!???.???,?!??,?" Also QDT is not happy and cheerful, they are very mischievous and a prankster hehehe, if they were ever in a show or movie they would definitely be the villain. Look at them, they look so dead inside and evil!
