For example, say I went through all of the computer science courses on Academic Earth (http://academicearth.org/subjects/computer-science) and also used the Khan Academy (http://www.khanacademy.org/) to bone up on my math (I already have a decent background in algebra, probability, statistics, and calculus). I am also learning Python and Django and know HTML, CSS, and some Javascript. Plus there are a lot of great communities online that I could go to for help, and I would probably make a lot of great friends there too.
Other than the people I would meet and the actual degree, how much would I actually be missing out on if I went the DIY mentioned above? What, specifically, would I not be able to understand other than strictly theoretical stuff (machine learning, NLP, etc.)? Would going back for a master's degree in computer science really be worth it (perhaps the MCIT program at UPenn: http://www.cis.upenn.edu/grad/mcit/)?