There are other extras, based on dressing like the characters, how they learned the dances, a sing-along karaoke section, etc. My two favorites are the Glee Music Jukebox, perfect for those who are just enamored with the songs, and the Fox Movie Channel's Casting Session. The casting session segment is very interesting, and really brings a lot of depth to one's understanding of how the show came to be. Gary Newman, Chairman of FOX says, "The idea of a musical on television, well, it's just rarely been done and almost never successfully," when talking about how the show was pitched. I thought Kevin Reilly (President of Entertainment at FOX) had a perfect explanation of why the show works as a musical: "That is the distinction between doing a musical and a comedy with music. This is not something where characters spontaneously break out into song. They sing in an organic setting... it is very sweet and yet it is snarky."
So for people who've never seen it, rent it... or rent the first disc or two and decide if it is your cup of tea. If you're not sucked in by the end of the first disc (first four episodes), then the show is probably not for you. But if you like it, you'd do fine to consider buying the hearty seven-disc set and take your time savoring it. And if you already consider yourself a Gleek, then hey, do something nice for yourself (you deserve it), and get Glee: The Complete First Season to enjoy again and again. Maybe you can even lend it to friends and bring them into the club.