Delta also plans to increase the number of "tiers" in its SkyMiles plan this spring. Instead of offering domestic tickets for either 25,000 or 50,000 miles -- with many more tickets available for 50,000 miles -- the airline will offer 50% of its total seat inventory for 40,000 miles, while maintaining the same amount of seats in the 25,000-mile tier. Most remaining seats will fall into the 60,000-mile tier.

This is better than 25/50, but 50 isn't always available.