it's hard to pick "most surprising", so I'll mention a few things that stand out as having surprised me:

• that bitcoin actually works, and has continued working, from genesis with one user to today with millions of users.
• how much Satoshi got right when he launched bitcoin. in retrospect and compared to everything that came after, that is quite amazing.
• how difficult it is to use bitcoin privately. simply generating a new address and using Tor (as Satoshi suggested) is far from sufficient. I'm convinced that adoption of some kind of zero-knowledge protocol like Zerocash is needed to give users both "dummy-proof" and "future-proof" privacy.