Dollhouse is in many ways Joss Whedon's most challenging and most cutting edge show, trying to deal with issues that are rarely or never addressed on television. With low ratings making a third season unlikely, Joss Whedon and his writers packed the second and final season with several seasons' worth of story arcs, resulting in one of the richer narrative arcs found on TV.
As painful as it was the time, getting cancelled (and more importantly, knowing that the end was almost certainly coming) was the best thing that ever happened to Dollhouse. Whereas Firefly was pretty clearly nipped in the bud and became a classic example of network-squandered potential, Dollhouse got that second season. That still only brings the total number of episodes for the show to 26, but given the pretty horrible ratings the show had to begin with and Whedon's perennial refusal to compromise on quality, once that second season began Whedon and company knew they had to move fast.
Dollhouse is the hardest to synopsize of any of Whedon's shows; normally I'd direct you to the Wikipedia entry for a brief refresher, but the description there is so filled with spoilers, and so much of the joy of watching the show live with friends (particularly once Fox switched to two episodes a week when burning off episodes the end of the second season, which is when it REALLY became must-watch TV) was having the densely packed and often surprising revelations that the show launched at us steadily blow our minds. The gist is that the Rossum corporation has developed technology that allows them to treat human personalities like computer programs. Sit in their chair and they can back you up to disk, erase you, even outfit you with a whole new set of memories, mannerisms, and skills (in a neat wrinkle, these personalities aren't pick-and-mix; your hostage-negotiation skills, for example, might come with poor vision).
What they choose to do with this technology is like a skeevier version of Philip K. Dick's short story "Paycheck": You go to sleep one day, and then you wake up and it's five years later. You get paid a lot of money. And for those five years, your body has been an "Active," kept in a, well, doll-like state between assignments and regularly sent to perform tasks from assassination to prostitution to simple companionship. More importantly, when you're an Active, you're sent to live a life; it's technology that lets you see your wife one last time, or solve your own murder, or find out where that serial killer in a coma on the other side of the room put his latest intended victims. Not only do "you" (your personality) have utterly no idea that these things are happening, "you" (your body) really IS whatever it's been programmed to be, with utterly sincere emotions and reactions and no inkling that you're anything other than a real, normal person.
As you can imagine just from that sketch, it's a very rich setting, probably the headiest, densest science fiction premise seen on network TV in years (not that many people noticed), and as you might expect from Whedon, the show refuses to back away from any of the difficult, sometimes disturbing ramifications of the ideas that Dollhouse is based around. It's just that these issues and this setting are overlaid with the typical Whedon mix of snappy patter, romantic angst, and plot whiplash.
It's not that I'd want Dollhouse to be more serious-minded or something; the show is commendably dark and serious when it's called for (as with all of Whedon's shows, the tonal and emotional range here puts most network TV to shame), and the chances that it takes in terms of plot, theme, even character are usually commendably well-done. And once the show's awareness that it didn't have long to live meshed with the accelerated airing schedule required by low ratings, Dollhouse hit a truly gonzo high, seemingly rushing to get out five or six years of serialized TV in just a few episodes.
The infamous, not-aired-in-the-US "Epitaph One" that ends the first season was ample notice that Whedon wasn't relying on Lost-like longevity to get his whole story out, and while everyone I know who's watched the show can pick out a few moments that just register as goofy (albeit usually different moments for each person), that's a small price to pay for one of the few genre shows to go down with the ship, so to speak; to stick with its thorny premise to the (literally) bitter end, to take the kind of potentially apocalyptic shift that serialized TV usually just pussyfoots around with and have it lead to... well, apocalypse; to push these characters until some of them snap, and to show what happens then...