For me, there were two things that surprised me the most, despite reading and hearing about both.
First, I was amazed at the change in hunger sensation. I was (and remain, for the most part) simply not hungry as much anymore. Pre-op - and even at the start of the liquid phase - I felt hungry all-the-time. For me personally, it was one of the biggest contributors to my past diet failures. The "surprise" was a pleasant one, to say the least. It is so refreshing to feel like food is no longer controlling a significant portion of my thoughts throughout the day.
The second thing that surprised me the most, was how little food it took for me to feel full, and how quickly I felt full. True, this was discussed with my healthcare team, and I read about it here and elsewhere, but I don't think I was truly prepared for just how little it takes for me to feel full. Again, another pleasant surprise.