I think part of what made Sheila appealing was that she had a fuller range of emotions compared to the rest of the cast. She worried about her kid brother. In moments of pathos she shed a tear. When I see 80's era American cartoons nowadays sometimes they strike me as emotionally deflated. The committment in that era to "kid friendly" programming bled a lot of the oomph out of the shows. Sheila may have often been the stereotypical "overemotive" female, but at least she had emotions. And I'd be lying if I didn't admit that her emotional vulnerability was part of the character's appeal. But then I also get a thrill out of the Savage She-Hulk beating up robots, so I'm kinda all over the map when it comes to my tastes in imaginary chicks.
Then there was that one incident that forever made Sheila special:
Truth be told this sequence only takes a split second in the show and I don't remember it from my youth. I didn't really see this until ABC Family started re-broadcasting D&D a few years back. My first reaction was "OMIGOD! Sheila panty shot!!!!1!!" My second reaction was "Dude, you are such a perv. She's like sixteen or something." In my own defense I would note that when I started crushing on Sheila she was a sophisticated older woman.
When someone posted that screencap on theRPGsite it called to mind this painting by fantasy T&A guru Dorean Cleavenger called 'The Courtyard'. It's one of his tamer works:
When I first stumbled across this pic, back in the early 90's, my first thought was that this picture was Sheila all grown up. I think it's the combination of red hair, exposed thigh, tall boots and cape that caused this reaction.