Are there any notable voice actors who only play "serious" characters and never any comedic characters? Peter Cullen comes to mind, as i dont recall any comedic roles from him (he did play Mighty Man in Mighty Man and Yukk, but he was the straight man of the duo).
What about Eddie Spencer from Filmation's Ghostbusters? What about Murky Dismal, Rainbow Brite's (not-so) arch enemy (although I guess you could call that one "seriocomic"...)?
Anyhow, I'm sure there are others, even if I can't think of any.
Yeah, Cullen originated the voice, but Cummings took over about halfway through the series (although it's almost impossible to tell the difference between the two actors).
Tony Jay has definitely done comedic work before, though. Rugrats and he played a comedic role in King's Quest 6... as well as a few other things I believe.
Now Michael Ironside... I can't imagine has done any comedic work.
He also did the voice of Father in Codename: Kids Next Door. Admittedly, he started out as serious and menacing, but gradually degraded into comical buffoon, ala Jack Spicer.