It is hard to say about now, for most of the interesting AIX features are tightly coupled to the IBM hardware (e.g. dynamic CPU re-/allocation across LPAR's and

It is hard to say about now, for most of the interesting AIX features are tightly coupled to the IBM hardware (e.g. dynamic CPU re-/allocation across LPAR's and a gazillion of other things), and the hardware is rare to come by nowadays. For its time, though, AIX was highly innovative and was perhaps the first UNIX that bundled a LVM and a journaling file system into the standard system distribution (and not as separately purchased commercial products). It also has a single set of .a libraries which are both used as dynamic AND static libraries (courtesy of the XCOFF object file format). It also has uniform and very tidy naming conventions for command line utilities, e.g. anything that can create something (e.g. a user account, a user group, a logical volume etc) is prefixed with mk-, mutating commands are prefixed with ch-, destructive operations have the rm- prefix. Pervasive performance metrics and counters across the entire system are also cool. Lots of other small niceties I have forgotten about.