I miss the people from my first job (that I hated). Although there were a few duds, I learned a whole helluva lot from most of the colleagues I worked with being a theory-heavy / reality-lite recent grad engineer.
On my second, current job, I will miss feeling being a part of something bigger,that it is someone other than me making the critical decision.
In general, I'll miss being treated well, with deference and respect. I'll miss having first class insurance and retirement benefits (in addition to the pay and bonus). And I'll generally miss waking up in the morning and looking forward to getting to work. Resenting the idea that work is getting in the way of being more healthy, but I'll have a salad at lunch and run 45 minutes in the afternoon.
I'll miss the easy cadence of life in the big picture. Although I'll also be OK without it. It's just nice to be a part of what I see happening in the world (I was just in South Korea talking to folks on the street about their President being arrested, North Korea, and how they feel about Trump), but I do understand how easy it is when you retire and have your own little routine also.