Things I miss:
1) Jails without having to have a hypervisor system, Linux doesn't really have anything comparably as easy to use particularly with the iocage project managing the jails.
2) ZFS out of the box supported at at lower level than modules and user tools. ZoL has come a ways but Linux's political opposition to it is always going to be a weight around its neck.
FWIW I am using a ZFS pool that has never been wiped/recreated since FreeBSD 8. It was created on that OS and has been migrated between Linux/BSD twice over a span of 11 to 12 years.
Things I don't miss:
1) The lack of file/directory monitoring libs and third party applications because, honestly, kqueue sucks.
2) While simple SysV init was simple 15 years ago systemd has come a long way. I hate to admit it, but systemd is the better way at this point because I like having my daemons restart themselves without third party monitors.
3) Brain drain is real, it seems the last time I used FreeBSD that port maintainers are running thin and I don't have a suggestion for how to attract more of them. For many years FreeBSD's package/port system was the best in the world (yes, better than the Linux alternatives like apt and rpm too). I think pkgng's rollout was bungled, though, and while it may be better now it seems to have a lingering cost in port maintainers.