I miss /usr/ports. I could spend days just exploring its contents.

I miss an /etc structure that wasn't a complete mess.

I miss UFS and its soft updates.

I miss the stability of fBSD 3 and 4.

I miss the ease of which you tweaked, compiled, and installed a new kernel.

And just because of the hilarious legacy that was obsolete 20 years beforw I started with it, I miss the concept of font-servers.

The main reason for my migration was the bigger userbase of linux where it was easier to find people who has resolved whatever issue I was having, plus nvidia drivers. Plus I've only needed to use fBSD once professionally.