I thought this write-up largely got it right about the form and format of Sheldon (from The Webcomicker):
Some comics never let their characters change or grow. And for a lot of strips, that’s just fine. If you’re doing a gag-a-day, or some light-hearted romp, you can have your characters stay pretty static. And it is possible to create well-rounded, interesting, complex characters that your audience connects with and have them be completely static characters. Sheldon is a good example of this. There’s a lot to the character of Sheldon. He’s a genius, and a huge nerd, and struggles with not having “real” parents (although not too much, it is a humor strip, after all). He’s kind-hearted and generous, and also struggles with awkwardness and occasional feelings of inadequacy. He’s a fully-developed character you can relate to. But he’s definitely static. The Sheldon you see in today’s strip is pretty much the same Sheldon we had at the beginning of the comic. And should the strip continue for twenty more years, it’ll still probably be pretty much the same Sheldon at that time, too. And for a light-hearted humor strip, it works.