An interesting question for me to ponder since I am new to being an elite. (OW Emerald and *A Gold)
I am basing this on my OW expiriences on AA, IB and BA and recent status matching on CO with flights limited to interEuropean LH.
OW Lounges: A+ BA lounges are simply the best at LHR. BA lounges at LHR are far better than the comparable LH lounges at FRA.
Network: B Huge holes in Africa. I also think AA leaves some areas underserved in the US, mostly in the Southeast. When flying from CAE or CLT to Europe why do I have to fly to DFW or ORD when there is a flight from RDU to LHR?
Reciprocal Bennies: A I feel that I am treated very well on other OW carriers, to include occasional Op-Ups.