Posted my thoughts on the overexposure of Elmo in the retired Muppets list thread before i discovered this one (quick summary - it's not so much that he's been overexposed but it's been his personality/design changes that have annoyed me in the past decade more than his screentime), so here's my list...with one exception, nothing that hasn't been mentioned already...
(in no particular order)
Herry - not only do i miss him, but i miss his old design but i miss some of his older personality traits (clumsiness, his simulaneous love for things like barbells and dollies, being a bit more initially intimidating than other monsters but completely sweet once you got to know him - that to me defines the essence of what the Sesame Street monsters - well actually all the Muppet monsters are about)
Guy Smiley - he's made baby steps out of retirement but let's see him more of the show proper and in the higher profile specials/video released.
Benny Bunny - way too classic of a character that was a casualty of the show's getting back to its roots and away from the Around the Corner crowd in season 30.
Frazzle - but he needs to be brought back in his original characterization - he did enjoy a revival around the early 90's (as did a lot of Jerry Nelson's characters) but they started to ruin his character by defining that he was a "younger" monster than the others and that his monsterish-ness was more a result of his younger age.
Lord Chatterly. Again i'm thankful that he got a temporary revival in the early 90's with the resurgence of Jerry Nelson characters since that was a character i thought would NEVER be brought back at all - but i would love to see him yet again.
(Honorable mention at place #6
Don Music - i know the child psychologists/PC crowd at Sesame Workshop don't like him, but he could be brought back without his head banging tendancies (even with his "I'll never get it right - never" catchphrase just without that last "punctuation") and done well by someone like David Rudman. In fact i would go so far as to say he could be a valuable part of the curriculum by using him as an example of how frustration is a legitimate and valid emotion and how it can be dealt with in different ways - showing Don's defeatist tendancies which is then countered with other characters in the Kermit role (if not the frog himself) showing him that he can still reach his goals if he doesn't give up.