Not much more to say on this than -- he will definitely be missed. I know that obituaries and tributes are going around the net today so I'll leave all that to the others.
For me it wasn't only his writing and illustrating that made him stand out as an author and a person - it was the fact that he spoke his mind, and didn't talk down to children. He did scare the pants off me the first time I read Where the Wild Things Are, but frankly, I got over it just like most kids did (and do). He didn't worry that his readers would be scarred for life or that we weren't able to handle ideas that were for adults.
Thank you, Maurice Sendak, for all that you did.
And for those geeks like me -- here's the basic Wikipedia potted biography of Maurice Sendak, just because:
Rest in Peace (or... in a wild rumpus).