In terms of a truly seamless experience I think there are far more important ducks to get in a row before elite benefits parity.

Pricing they seem to have nailed if you look at BA/AA offerings and TATL codeshares (Not that it is OneWorld's doing), but what about consistency of product on such routes?

Customer service. You have a problem with AA or BA then you normally get it fixed, you at least get a sympathetic ear. LA and IB you may as well go and piss into the wind. Cancelled flights without warning, inability to fulfil elite packs, regular delays without caring, and pity the man that has IB or LA lose his bags. This I think devalues the OneWorld brand more than anything, particularly when OneWorld has no teeth and won't ensure a bit of brand consistency and fair play.

Reservation systems pick ONE so that passengers ticketed by one airline can manage their booking on the operating airlines. Now LAN is moving over to SABRE too so this will only be exacerbated, apart from LA/AA ticketing/operating scenarios.

AA has a policy that if you are travelling on separate PNRs for two different OneWorld carriers and AA causes you to miss your connection for whatever reason, AA will be accountable for rescheduling your connecting flights with the other carrier. Make this policy across the whole alliance and enforce it.

Write, publish and enforce an interline baggage policy between OneWorld carriers. Guarantee that bags can be interlined through separate PNRs on separate carriers and make it work. The only thing that is stopping this happening is miserable check in agents that refuse to do it.

I'd personally take any of the above over priority boarding.