Damnit. He wasn't just a great character designer/artist; he was a great person, too. I actually asked him if he would fix Ripjaws after Ultimate Alien ruined his design, and Derrick personally promised to me that he would - and he actually did fix Ripjaws in Omniverse. His designs made Ben 10 cool/fun again after Ultimate Alien/Alien Force sucked all the life and humor out of it, and his designs for Transformers Animated are still the best character designs Transformers has ever had.
Also, I used to be in a roleplay on Formspring, and he would joke around with the fans and make silly things happen to us. He was a genuinely wonderful human being, and I wish I could have gotten to meet him in person.