Still, overall I call it a failure with a couple of effective moments. Spoilers.
First, the stuff that worked. The three actors were terrific. The roles weren't all interesting, but they put a lot of charisma into them.
Second, Harris was fantastic *and* terrific. Really. Except for a couple of awkward, let's-improvise-this moments at the start, he put a lot of heart and spirit into it.
And some of the story beats worked just the way they were supposed to. The end of Act 2 was the best part of the whole show. When Dr. Horrible, after being humiliated by Captain Hammer, breaks out in a murderer's smile, it's a moment that works. Especially coming right after the hammer/penis joke.
But still, there's a lot that fails here, and the biggest problem is Penny. The actress has a lot of charisma, and if I was going to do make an icon out of the show (as so many others seem to have done) it would be her holding the spork as she says "Huh?" It's a beautiful, perfect moment.
But who is she as a character? Your basic idealized romantic interest and nothing more. She doesn't get to have odd traits, or unpleasant habits. She's just gentle and generous and idealistic and empty. She's there to be admired, and to give us hope that she can turn Dr. Horrible into Dr. Has-His-Shit-Together.
Why not give her a bad habit? How about a goal she can achieve on her own instead of one the bad guy hands to her? How about something small, like a quirk?
This means that, at the end when she dies, it's just the death Dr. Horrible's chance at redemption that we're seeing. She's hardly even real. It's boring. It's bad writing, and it deflates my interest. I want to see real people, not paragons, even in goofy superhero musicals. Especially in goofy superhero musicals.
And there's the whole "Murder someone and get into the Evil League of Evil" (or whatever it's called) plot line. Jeez, is that the least exclusive organization since the McDonald's Club? It's even easier to get into the ELE than it is to get into HWA. I could use a mutant power like, I dunno, gun-purchasing and then randomly shooting people to qualify.
It just doesn't work.
The music was mediocre, although it was sung with enthusiasm. In fact, in some places (like the end of Act 2) the music dragged out a plot point and stole its power. And Captain Hammer was too broadly drawn. He didn't match with Penny, Dr. Horrible or even Moist Man, who were all much less exaggerated.
Finally, the switch from 3 minutes of video blog (and with a title like "Dr. Horrible's Singalong Blog") expected it to be a blog. It was jarring when they switched to a TV show format, with studio lot exteriors and such. It was disappointing, actually, as though they couldn't stick with the format.
Meh. I thought the end basically worked, considering the way the story was going, but I was only half-interested by then. In fact, it was a good ending, and would have been much stronger if the story leading up to it had been solid.