I've flown about 20 segments on Delta Airlines from short haul flights to 10+ hour transatlantic flights. In-flight service usually is not a great factor when I choose an airline. But in you ask me, I think their level of service has been on a downward spiral since the early 90's. Their short haul service is average, seats are somewhat tight and their meal service could use much improvement, but the flight attendants are generally pleasant and professional.
I've flown on Delta's 727, 737, 757, 767, L-10-11, MD-11, and MD-88 aircraft. In terms of performance, the L-10-11 was my favorite, but the 757 and 767 are not far behind. In terms of passenger comfort, I think the 767 and 757 were the best. The 757 is the aircraft I've flown on the most, usually on segments lasting about 2 hours. They are great planes, though they might get uncomfortable if you are on a trans-con flight. I've also flown on Delta's 767's several times, the 2x3x2 seating is good, and the flights I've been on have been standard.
Delta's transatlantic service has made a poor impression on me. As I said earlier, the level of In- flight service usually is not that important to me, just as long as I reach my destination safe and on time. This flight was an exception. I was on a Delta MD-11 flight from Atlanta to Rome. To avoid the long story, the flight was delayed over an hour, the gate agents were like cattle herders, the plane was like a sardine can, the flight attendants seemed to enjoy bumping the meal cart into my elbow, and toilet paper was strewn in the aisles. I'm hoping this was an isolated incident.
In terms of connecting airports, Atlanta is set up well, it just can get very crowded, I would suggest making sure you have enough time to make you connection if you are traveling through Atlanta.
Cincinnati is also a good connecting airport. It's generally not as busy as ATL, but can also get crowded at times.
I have not flown through Delta's Dallas or Salt Lake City hubs, so therefore I cannot give my opinion on them.