1996 was the first election in which the Internet played a major role. In late 1996, roughly 20-25% of American adults were connected to the Internet. Adults with internet connections were significantly wealthier and more educated as a whole than those without. (
Internet Technical Resources: Internet Population Estimates)
By 2000, about half of Americans were connected to the Internet.
By 2004, about two-thirds of Americans were connected to the Internet, and it was the first election in which blogs played a major role. I also believe that the 2004 election was the first to be covered in high definition.
2008 was the first election in the era of social media (Youtube, Facebook, etc.), which candidates pursued. However, social media was not nearly as ubiquitous in the life of the average American as today.
In 2012, the majority of Americans have Facebook accounts and are able to access them from their phones, often at high speed. Along with Facebook, Twitter is a major means of communication between candidates and voters that was barely used, if at all, four years ago.
Anything else?