This is a very interesting topic. I think that many people have wrong expectations but I won't talk about that, I'll talk about what was my expectation before I got to know Bitcoin well.
Let's start:
1. I thought that Bitcoin was an anonymous currency but later I found out that Bitcoin is not an anonymous currency because we have blockchain where every transaction is recorded and available publicly for the audience, which means that everyone can find every transaction.
2. I thought that Bitcoin was a cheap payment method but I was wrong. There are moments when Bitcoin transactions are cheap but there are also moments when Bitcoin transactions are extremely expensive.
3. I thought that Bitcoin mining was easy and possible but I was wrong here too, Bitcoin mining is impossible at this point if you don't own a huge mining data center with cheap electricity. It's impossible to mine at home via CPU/GPU.
There are probably other things that didn't meet my expectations but it doesn't come to my mind right now. By the way, I also want to talk about what met to my expectations:
1. You are your own bank | This is true because when you own Bitcoin in your wallet, there is nothing and no one that can stop you from owning it if you take security seriously and aren't careless with your wallet.
2. Not your keys = not your coins, this is also very true.
3. Bitcoin is a good P2P payment method that really solves that problem that's written in its whitepaper.
EDITjohn_egbert, you were very fast
Thanks for correction