My advice is that when you compare programs, don't look at the mileage redemption requirements in isolation. Consider the total cost (miles + cash) of an award ticket from the program.

Imagine that you want to book a ticket on Delta SkyMiles (the industry's undisputed leader in innovative "gotcha" fees) to Puerto Rico two weeks from now on Continental Airlines. In addition to inflated mileage requirements, it's going to cost:

$75 for booking within 21 days of the flight
$25 for booking on a partner airline
$25 for booking over the phone (which you'll probably have to do since the web site is awful)
$50 fuel surcharge

That's $175 (!) just to get started, and heaven forbid you need to make any changes later to that "free" ticket.

On the other end of the spectrum, Southwest Airlines is honest and straightforward. Need a trans-continental award ticket a few hours from now? No last-minute booking fee, no telephone reservation fee, no fuel surcharge, and no change fees later on.

Know which kind of airline you're dealing with.