Good question, I will be on JAL's codeshare on Monday.

Being an asian airline, many things already are better than AA, I feel.

AA codeshares with most JL flights from NRT to SFO/LAX/ORD/DFW and HNL. I've taken their F class and will be on J soon.

Aircrafts - Transpacific: Mostly 747s (both -200 and -400 series).

Seats - Flat beds in First. Some individual seats, some pair seats depending on aircraft.
Business has their "Seasons" J class seats of 50" pitch. Apparently JAL changes the colors of the seat fabric according to the season itself but as I have not flown on them consistenly, I cannot confirm that.

Economy class is basic - but if compared to AA's MRTC JL's seats are 'tight'. Overall service is better than AA's.

ON AA codeshare flights, miles are apllicable toward status bonuses. But flights on JL flight numbers are not, though they post quite fast.