It is a modular server OS (think RISC) rather than windows server OS (think CISC) with its initial (and core) feature being network attached storage server. So the overriding "feature" is that it is modular and can be customized to the needs of the owner. As many videos say if only the storage server component is used then TrueNAS is a generally better choice with sufficient RAM - its stays closer to its intended purpose and is free. Adding modules does add to the strain on the hardware and management and if too many are added then the owner would find the complexity would be to the level of windows server and that then windows server would have been and will be a better choice - an OS designed to be complex in the first place is more efficient than and OS designed to combine modules.