The main actor and actress, as well as the best program, are divided into three distinct genres, and they should be. Just as comedy movies are almost completely ignored by the Oscars, you won't catch the Golden Globes giving a supporting performance trophy to a sitcom star very often, if ever. It's completely unfair to put them together, even when effort is made to divide the nominations between all three arenas.
However, this separation has some trouble, too. Calling the comedy category "Comedy or Musical" comes from an era when all movie musicals were light-hearted, thus not belonging in the heavy drama world. That might work for Glee, but Smash and Nashville are every bit dramas, and should not be tossed in with the half hour situation comedies.
Those aside, you used to be able to tell if a series was a comedy or a drama simply by its running time. Dramas are typically hour-long shows, while comedies are half hours. But as HBO and Showtime have entered the arena with Nurse Jackie, The Big C, How To Make It In America, Weeds, and Enlightenment, this is no longer the case.
The problem becomes in how you define these types of series. I look at comedies as programs where the characters act a certain way to get a laugh, and the situations arise just to make the characters do funny things. If there are some jokes, but the main point of the story is an ongoing personal experience that fundamentally gets to the core of a person and forces them to look at themselves, not a humorous device, then it's a drama. I sometimes laugh at Nurse Jackie, but the show is about Jackie struggling with addiction and lies. This is not a comedy.
The Golden Globes haven't gotten wise to the shift, though. Which is probably why these types of series have come in and stolen the trophies from more traditional fare on the broadcast networks.
This year, Lena Dunham's win for Girls kind of throws a wrench into the entire thing. Her series is definitely drama, because serious things happen, and self-reflection is a large part of the story. Yet, Girls provides many more chuckles than its premium peers, and the arrogant, self-involved way in which Dunham's character behaves is inherently comedic. It's really, really hard to put this show in either box.