I've been trying very hard to find the right words to express this...
His voice is irreplaceable and we'll all miss it. His talent was undefinable, and that will be missed too. While he did have a lot of star characters, what I like to remember about him is the fact he was so versatile that he'd do more incidental characters than any other puppeteer. While some like to dub him the Mel Blanc of the Muppets, I actually liken him to the ever versatile Paul Frees. That is, in Rocky and Bullwinkle, his only main characters were Boris and Inspector Fenwick (as well as recurring characters, such as Peter Peachfuzz) but he played almost every single incidental role in all the other cartoons.
Saying that he'll be missed is an understatement. He was one of the first of the Muppet performers after all, a founding member when the group went national. He really got to shine in Sesame Street and became a staple of Muppet projects ever since. We lost a link to the beginning of the group, and that's pretty depressing itself.
Thankfully, he is immortalized by all of his outstanding input on all of those wonderful Muppet projects.