Int'l C/J or F award on a really good airlines (think CX/SQ ) is the most important to me.

ditto

ease of getting award tickets has to be tops with me. Fewer hoops to jump through make me a happy flyer.
Nick

I'm big on the free travel, so the rate at which that can be earned (with bonus miles due to status being a big part of that) is probably #1. OTOH, an LCC with generous in-network awards still would be highly unlikely to steal me away from a global alliance because I want the destination selection. Upgrades are also good and will break any tie, though I never spend miles on them.

All in all, I think people will fall into either the "upgrader" or "free-travel" camp, and I'm in the latter.

Most important? It's really difficult for me to distill a program down to one benefit.

I used to be an "upgrader" as RustyC mentioned, on AA. Earning miles (on most every fare -- that's important) and redeeming them for upgrades, not free travel, was the most important thing. Since AA had decided to charge a $500 roundtrip upgrade fee, plus the miles of course, I'm now a "free traveler" on AA. Instead of redeeming 50,000 miles + buying a ticket to get into international business, now I'll just redeem miles for a free ticket and AA can forego all of the revenue from that trip.

Elite benefits are also very important (esp. priority standby and free/cheap domestic upgrades).

So, yeah, that's not one most important factor. Sorry.

For me it is upgrades and knowing the airline will be around when I want to use my miles (I include United in the group who will be around)

Award availibility is #1 for me since I don't fly enough to get status