I like when storytellers treat unlikely heroes without overly exploiting those characteristics that make them unlikely. Sure, it's kind of hard to get past what makes Sloth "unlikely", well, because...he's some kind of mutant.
When writing a character that serves as an unlikely hero it's easy to focus on those traits that make him unlikely. I suppose it is more difficult for writers to give said unlikely heroes a totally fair treatment, maybe it's a matter of difficulty identifying. Still, I think media is becoming more accepting of these unlikely folks as characters.
Sorry if this was lame, I couldn't really think of anything else to write. That...or I just wanted an opportunity to talk about sloth. Sloth is awesome, he can rip out your vas deferens and strangle you with them.