That people knew what they wanted.
For the longest time I thought I would talk with people, they would describe a problem or workflow and I would put it into code and automate it. Turns out every time that happens, what they thought they wanted wasn't actually what they wanted.
Edit: I agree with most of the comments. This is not a technical answer and may not be what the questioner was looking for. It doesn't apply only to programming. I'm sure it's not my longest-held assumption either, but it was the most striking thing I've learned in the 10 short years I've been doing this. I'm sure it was pure naivete on my part but the way my brain is/was wired and the teaching and experiences I had prior to entering the business world led me to believe that I would be doing what I answered; that I would be able to use code and computers to fix people's problems.
I guess this answer is similar to Robin's about non-programmers understanding/caring about what I'm talking about. It's about learning the business as an agile, iterative, interactive process. It's about learning the difference between being a programming-code-monkey and being a software developer. It's about realizing that there is a differnce between the two and that to be really good in the field, it's not just syntax and typing speed.
Edit: This answer is now community-wiki to appease people upset at this answer giving me rep.