What makes an idea viral?
For an idea to spread, it needs to be sent and received.
No one "sends" an idea unless:
a. they understand it
b. they want it to spread
c. they believe that spreading it will enhance their power (reputation, income, friendships) or their peace of mind
d. the effort necessary to send the idea is less than the benefits
No one "gets" an idea unless:
a. the first impression demands further investigation
b. they already understand the foundation ideas necessary to get the new idea
c. they trust or respect the sender enough to invest the time
This explains why online ideas spread so fast but why they're often shallow. Nietzsche is hard to understand and risky to spread, so it moves slowly among people willing to invest the time. Numa Numa, on the other hand, spread like a toxic waste spill because it was so transparent, reasonably funny and easy to share.
Notice that ideas never spread because they are important to the originator.
Notice too that a key dynamic in the spread of the idea is the capsule that contains it. If it's easy to swallow, tempting and complete, it's a lot more likely to get a good start.
But that doesn't mean that there's no role for mystery or ideas that unfold over time. In fact, the unmeasurable variable here is style. Howard Dean's ideas spread at the beginning--not because of the economic ramifications of his immigration policy, but because of the factors above. The way they were presented fit into the worldview of those that spread them.
A key element in the spread of ideas is their visual element. iPods and visual styles spread faster in the real world than ephemeral concepts. Pictures and short jokes spread faster online because the investment necessary to figure out if they're worth spreading is so tiny.
And of course, plenty of bad ideas spread. Panic, for instance, is a superbad idea at all times, but it spreads faster than most. That's because spreading an idea is rarely a thoughtful, voluntary act. Instead, it is near the core of who we are, and we often do it without thinking much about the implications.