WN is obviously an important carrier, and the principal reason I excluded them is that data is hard to come by since their recent conversion from an "any available seat" to a "limited availability" award system. They also only allow folks who have already qualified for an award ticket to search award inventory (I won't qualify for my first "new" WN award ticket until next month, so I've never been able to search). The anecdotal (and preliminary) information from the WN board would suggest domestic availability is at least as good as AA's or UA's.

That said, there are obviously some significant limitations in WN's program over those offered by these other carriers. The biggest limitations are that you only get 2 years to earn your tickets and, once you do, the redemption clock starts ticking: 12-months "use it or lose it." Given these limitations -- which obviously limit the number of free tickets out there -- it would be pretty shocking if WN didn't offer better-than-average availability.