They have the worst redemption options for award tickets in the domestic market. If you fly internationally, Skyteam is a pretty good alliance that will give you options, but if flying domestically (or internationally on a Delta operated flight) and you are wanting to actually use your miles, then it can be tough.
This especially holds true when trying to redeem first/business class seats which give you the most value per redemption.
Not all is bad at Delta - As missnd posted, if you value upgrades on paid fares, then Delta may be the best domestic airline for you (although you have to be gold and really platinum before you will see a lot of upgrades).
Agree on checking out flyertalk. Lots of good information there.