Compared to the other major US airlines (SWA, UA/CO, US and AA), Delta is about the worst in terms of being able to redeem your miles and the amount of miles required for rewards tickets. Don't even get started on using them for international flights.

Delta has a three tier award structure and you will rarely find award flights at the lowest tier (ie 25k miles for domestic roundtrip) when others have availability at this level. Lastly, Delta has no other partners with whom you can redeem for domestic tickets. US, CO and UA are partners in Star Alliance and AA miles can be used on Alaska (or is it the other way around).