I would say that the most exciting feature of AIX 6.x has to the Live Application Mobility or put more plainly an ability to snapshot the run state of the application an resume it on a different physical machine. Of course this can improve the uptimes of applications dramatically while being able to avoid the downtimes for maintenance, hardware upgrades, etc. To give a little better idea how Live Application Mobility works from more hands-on perspective IBM has published a nice little introduction here.