There is so much about this that is wonderful and the more I think about it, the more I wanted to give some serious critique. I guess you could say I want this to become "more perfect."
1) The dialog is great.
2) Whenever Scoobs says "Raggy" in that voice, that's just 100% comedy gold.
3) The "I've been waiting for your Raggy" shot is pretty perfect visually. Also the merging with Shaggy's legs as the music crescendos.
1) I don't feel like Shaggy's head needed to random things. I feel like the absurdity of the whole situation and the creepy / funny factor sold it and it didn't need the "extra" push of having Shaggy's head do random things.
2) The shots where Scooby had anti-aliased white edges around his art... I wasn't sure if that was intentional silliness or not but if it was intentional, I think it's another aspect that wasn't necessary. Visually I think the whole piece could have remained dark and serious because the theme and dialog are so hilarious.