All man pages were well written, nicely formatted easy to read and almost all came with often valuable examples giving quick enough understanding to check usage most often. That has been absolutely the thing that I've missed other *nix systems since.
But there are too many things were done so nicely and made it nice to maintain with HP-UX that it's not worth trying to remember and list all. But unfortunately shell environment was not match to convenience GNU tools Linux had from beginning. That is without making effort to install (read: compile from source for quite long time) those HP-UX if that was allowed. With university computing center that no problem, but telco side it was big nono -- not without getting product owner permission first :/
But just an example Ignite-UX was one of my favourites with HP-UX. The simplicity using a one simple command with few options bootable DAT tape that could then be used to either recover whole running fully functional system or clone that developed system first to staging lab and then up to production with ease was great time saver major upgrades and migrations. None of the Linux bare metal backup systems I've tested have been able to recover exactly same disk layouts, usually LVM part is poorly done. As has been VmWare p2v migration tools also btw.
That Linux LVM that Sistina did first before Red Hat bought them, is implemented quite exactly what HP-UX had for some time then.