I've never gotten the whole Joss Whedon cult--I'm sure he's very clever, but the concepts behind Buffy and Firefly didn't do much for me. Now, you do a musical about a budding supervillain's quest for acceptance by his peers (one of whom is a horse, another a Thomas Jefferson impersonator) and I am there. (Is it possible I have never written about my affection for The Tick here?) No lycra, sadly (the low-rent outfits are fetish-free for me, aside from Captain Hammer's studly leather gauntlets), but the songs are catchy and the cast--particularly Harris--is a joy to watch.
I particularly enjoy Whedon's reversal of the good guy/bad guy dynamic; you totally root for Dr. Horrible, while ostensible hero Captain Hammer is an arrogant, self-obsessed bully. (I never got around to seeing Hancock, since the reviews were all so mediocre, but Hammer seems like a variation on the same basic concept.) Perhaps it's the musical-comedy aspect of the project that made the connection in my mind, but I couldn't help thinking of Broadway's Wicked, in which all the "good" folks in Oz are exposed as bubbleheads (or worse--far worse) and the "evildoers" are tragically misunderstood. (War on terror, anyone?) The fact that moral ambiguity can still do well at the box office bodes well in the era of you're-either-with-us-or-you're-with-the-terrorists.
PS. All the press suggests that your chance to watch for free is done and that you must now purchase the thing via iTunes (or wait for the promised DVD), but that's not true as of yesterday. Look, it's right here!