The following responses are base solely on my experience so please don't take it as the norm as I have no other comparisons to go off of...
1. Most people in the tech/startup scene will congratulate you. Most people outside of the above will ask why you sold your company. The reason for asking probably varies. I didn't expect that. It was weird at first to have many of my non-tech/non-business friends ask that question, whether it was people I was close to or random folks I haven't talked to in awhile. I suspect many are just genuinely curious why anyone would ever sell their company (maybe its not considered a norm).
2. Right after the sale, there was a transition period with the parent company and the team would go on to work for the new owners. That didn't quite pan out for our founding team, myself included. I left after a few months. From there, I went traveling for 6-7 months just to travel and enjoy life a little bit. The grind during the startup consumed a large portion (read: all of it) my life and I wanted to just relax for a little bit.
I thought traveling was the thing I wanted to do. And then it wasn't. Like a kid excited about a new video game, the excitement wears off and eventually you don't care. After 6-7 months, I didn't care for traveling anymore either. Doing the startup had changed me (for better or for worse) and I wanted to work on something else.
3. I didn't know if I wanted to do another startup, a project, or whatevers. I just knew I wanted to work on something else. This proved insanely difficult. While we were grinding on our startup, coming up with new roadmaps and new ideas were a cinch, there was always ways to expand on things. Going back to scratch, with a world of knowledge I didn't have prior to the first startup made things incredibly difficult. You start to dissect everything and half the ideas I wouldn't have thought twice about in the past, I now wouldn't want to do simply because I know a lot more and have a strong disinterest in certain ideas, didn't want to deal with certain aspects of an idea, or just simply didn't see a viable growth for such an idea.
It took quite awhile to even just come up with any idea (business or not) that I wanted to solve for myself, if nothing else. There was a moment where everything just came to a complete halt because I was in this weird limbo stage where I was extremely picky about ideas or just had no interest in anything. Following up on HN or any tech news was more entertaining than helpful if anything. Ultimately it just took a lot of time away from all this stuff before ideas started to flow again (ideas that mattered). In time, I had more ideas than I could keep up with, but it took awhile to get there (about a year).
4. How it changed my life. The learning experience from start to finish is impossible to measure. And I don't just mean the hard knowledge you walk away with from doing a startup. There are an incredible amount of subtle things you learn as well, that may or may not be correlated with the startup and what the startup does itself. The financial change also helped me realize what I really wanted and what I didn't want. I came into the startup thinking I wanted a lot of things. After the sale, I got an incredibly nice place, a new car, etc... but then realized one day out of the blue, I sat there quietly on my couch and realize I barely sat in it more than a handful of times. I had a nice TV I never watched (ever since I got into doing my startup, TV became non-existant and the habit stayed), etc... All these things I gotten that I thought I wanted were things I no longer cared about.
I did a full 180 and got rid of almost everything, instead choosing to adopt a minimalistic life. Especially after I read a blog post from a friend who did something similar. He sold off all his possession, cars, house, etc... and everything he owned fit into a single backpack. Nothing more. It was an incredibly sense of freedom. He then went traveling around the world. I had no interest in traveling anymore but the idea of an extreme minimalistic life really appealed to me at that point. And that's what I did.
Of course other things that vastly was life changing was getting rid of all my debt, my family's debt, giving my parents a chunk of money to enjoy their lives, etc...
5. For the most part though, outside of the above, things doesn't change as much as people think they do. My day to day is a slight variation of what it was before. I still hang out with all my friends like nothing changed. I still work day in and day out on the computer on new projects, etc.. I still eat out at the same places, and for the most part, even when I wasn't financial secure, I never thought twice about my spending habits so that part didn't change aside from not owning any debt. Overall, I would say the biggest change is probably my perspective on life (the kind I actually wanted) and I am fortunate enough to be picky about what I want to do with my time now.
Hope that helps.