I didn't think of Guys and Dolls, but yeah. I complained multiple times about Penny as I was watching. She is both clueless and uninteresting--why is Dr. Horrible interested in her in the first place?

I wonder if this is effect rather than cause, as he snagged two really interesting male actors who could easily handle whatever he threw at them, and a female actor that I, at least, have never heard of and was not particularly impressed by.