OW is definitely a step down from *A from an award travel standpoint. From my USA based point of view as someone who normally books C/J awards:
Cons:
The main problem for US based flyers is having BA as the major European carrier in the alliance. The fuel surcharges on BA TATL are completely insane which limits options from NA to Europe. Taking BA out of the TATL picture certainly lessens the "quality vs quantity" argument. Go price out any dull TATL itinerary on aa.com using BA metal and check out the astronomical fees/taxes if you have any doubts about this.
Iberia, Finnair and Air Berlin are options to avoid the fuel surcharge but they all have a tiny presence in the US, more similar to OS/LX/TK.
In Asia, while CX, JL, and MH are excellent they (+AA) have a much smaller TPAC and intra-Asia footprint than the vast OZ/NH/CA/AC/UA/TG/SQ (and now BR) options available via Star. Not sure how anyone can argue this point.
Routing rules on AA are not as favorable as US. Again I cant imagine that even the most shameless AApologist could argue this point. Simply take a look at the itins people regularly put together in the main US *A thread if you have any doubt. 90k US miles + nominal taxes for essentially mini-RTW trips.
Pros of One World and AA:
Booking process is less of a crap shoot than dealing with US. Phone agents are far more capable than US. Booking awards is easy and straightforward - words which would not be used to describe experiences dealing with US.
AA C availability on their own metal is quite good in my experience.
Far more functional website vs US
Overall AA/Oneworld are certainly decent but options are definitely more limited than US/*A.