Let us not forget that he does a great job voicing Skips in Regular Show as well.
Skips is my favorite character, actually. And not just because Mark voiced him.
But like I said, there is NO shame in voice acting as a career. I love Star Wars, but I think Mark is much more valuable as a voice actor, because he really has fun with it. I respect his decision to retire as the Joker, since it gets him a lot more room to stretch and do other great voices.
I LOVED his Buzz Buzzard and unnamed Badger characters on the new Woody Woodpecker show.
But more actors that just seem to disappear need to open up to voice acting roles. Look what a boon Family Guy was to Adam West. The younger generation is going to remember him for being the crazy Mayor of Quahog instead of Batman. And to think that it all stemmed from a Johnny Bravo guest spot. Butch Hartman and Seth MacFarline actually pitched an Adam West Adventures show to Cartoon Network. Same deal with Carrie Fisher and her character on the show.
I'm seeing more and more big name guest actors on various cartoon shows. George Takei in Adventure Time... Neil Patrick Harris... someone who's a huge star on one of the hottest sitcoms of the day has been on Adventure Time
and Batman: The Brave and the Bold. Paul Reubens as Batmite in the same Batman cartoon. Even half the cast of SNL does voice work... Kristin Wiig and Fred Armisand on The Looney Tunes Show, Bill Hader and Jason Sudekis on The Cleveland Show.
Voice acting was a great niche for Wayne Knight, Brad Garret (before Everybody Loves Raymond made him a huge star), and Patrick Warburton... people with very unique voices that could give a real cartoony performance in live action programming/movies that work even better in animation form. All three of them were on Buzz Lightyear of Star Command, actually.