It's true that Elmo's been around for longer than Kevin Clash. I remember when he was sort of a "baby" SuperGrover; he came out during the "baby" (muppets, flintstones, what have you) craze, which I never understood - arent Muppets childlike enough? I really did hate him then - now, not so much.
Personally, I'm not an Elmo fan for one reason - he isn't very real. I don't like it when performers whose audience is primarily children make their characters good examples rather than people with real conflicts. Barney is the most extreme example of this type of character.
Elmo's been scrubbed clean of temper tantrums, never forgets to say thank you, and is never rude - not like any toddler I know. I appreciate the educational aspects of Elmo's world, they work, but I'd rather show kids the reality of working through living in the real world - nobody wants to share (until they learn to,) sometimes people hit you, and you don't always get your way.