Stevie "Wonder" is a true talent we've seen grow up and blossom. We saw his beginnings on The Muppet Show where he started off small and took some time to be comfortable with doing different voices (when he began, most his characters sounded the same which is extraordinary compared to now when he has such range and diversity in doing voices). But one thing that was quickly evident from the start was his puppetry skills in terms of physical manipulation.
No wonder his first breakout star (Rizzo) was someone who barely spoke at first. Watching The Muppet Show when i was a kid, i used to get all gleeful and point to the screen whenever Rizzo popped up. Is it any wonder why he was cast as the hammy dog Sprocket in Fraggle Rock (you know you're an obsessed Muppet fan when you first start to type "frackle" when you meant to key "fraggle")? He was able to turn a character that largely interacted with a human and didn't communicate with words and make him a lovable star. My fave character from FR though was Marlin. That was Steve's "Zoot" or Frank's "Marvin Suggs" - a smaller obscure character that was just so zanily out there that you couldn't help but laugh yourself silly and wonder "where the heck did that even COME from?"
Besides being a skilled puppeteer/manipulator, a good Muppet performer is in essence an actor - and Steve truly "gets" his characters - be they original or recasts. The interview he gave to ToughPigs where he was talking about the philosophies between Kermit's evolution of soul compared to Rizzo really demonstrates how seriously he takes his craft. He doesn't just slip a cloth thing on his hand and put on a nice little puppet show...he cares very deeply about the integrity of these inanimate beings that he brings to life.
Lest that sound too serious, let's not forget his great comic ability - he can easily make us laugh to the point where we have tears in our eyes - much is made of the Dave/Steve dynamic but let's not forget the Steve/Eric dynamic - heck the guy has chemistry with just about everyone he works with.
Going back to how he gets his characters - he's said that one of his harder recasts to do was Beaker because unlike Kermit and Ernie who were already so complex and three-dimensional, he just didn't know from where in Richard he came from. However lest there be any question that Steve hasn't come to fully grasp and inhabit Beaker, i refer you to the Webby-winning Ode to Joy. That was 100% Beaker (or "600%"? Being that there were half a dozen of them?)
Music is an important part of the Muppets and while different performers have their own strengths and weaknesses, i think Steve is one of their best (and most underrated singers). He has a very "clear" singing voice and when i've worked with other singers that have that same quality i compliment them on being very "Steve Whitmire-ish" (even if they have NO idea what that means!) My all time fave Steve performance - The Music Just Keeps Rolling Along from Jim Henson Hour.
Even though he was one of those who were around during Muppet Show days, the fact that he started so young is a huge asset as he has the experience of Muppets-at-their-peak as part of his history but at the same time (unless there's some kind of health issue we don't know about) one would expect him to have another decade and a half of devoted top-notch Muppeteering ability in him. I hope he remains with the Muppets for a long time and retains the place he's rightfully earned as the star performer he is and that anyone dealing with the Muppets gives him the proper respect that the man has earned. You just don't mess with the frog - and ya don't diss "Stevie Wonder"!