I think OW's key advantage over it's competitors is the strength of it's members lounges. 3 core OW members in particular (BA, QF and CX) all have excellent lounges in both F and J when compared to their *A and ScaryTeam equivalents. Even OW's other big carrier AA offers fairly decent lounges when compared to other US carriers, which is great for those travelling within North America.
I also like the 3 elite tiers that OW offers, although I do wish benefits such as priority baggage was provided alliance wide.