One reason "blogging" would have been hard to describe was because it used a medium that wasn't usable/practical before. So let's look at some medium-related concepts that are expanding or will expand in the near future:
-Mobile technology. iPads, iPhones, Kindles, etc. Some people predict that we simply won't be using computers anymore in 10 years.
-Customization. As economies of scale become less important, and businesses gather more and more data, products and advertisements are becoming much more individualized.
Based on these, plus the fact that Steve likes AI, my guess is some sort of portable extra brain-perhaps something that can help you achieve the behaviors you want based on your personal psychology.
My description doesn't really sound awesome enough, but I can imagine something like a personalized second brain being absolutely amazing.