I've had 3 flights on the Delta 738s in the last month, and quite frankly each was worse than the next. Let me list the reasons.
a) Seat pitch in coach is pathetic on any DL aircraft, but the 738 just seems tighter. Maybe its the drab colorscheme.
b) That horrible non-porous thin seat. On a longer flight, you sweat your a$$ off. Literally.
c) Those annoying flip-down screens. It is impossible to sleep with those things flickering a few feet away.
In contrast, I enjoyed my one and only MD90 flight earlier this year. The IFE system was broken, so I didn't have to deal with the screens, but alas that meant that GTE Airfone system was also dead. However, I'll make that trade any day of the week.
The 738 in Shuttle configuration is a lot nicer. The seat pitch is an almost comfortable 34", so even a middle seat is bearable.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada