Info on seats is here from the DL rep. To fit that many they are definitely slimline seats.. hopefully more comfortable than what is in the 757 or 737-800s.
http://www.flyertalk.com/forum/delta...ce-update.html
Here is a snip
The 21 Economy Comfort and 139 Economy seats are the same comfortable model (B/E Aerospace Pinnacle) we’re installing on the A330 and 757-300 fleets. This seat offers an articulating seat pan for additional knee clearance and we’ve added extra foam padding for more comfort and a winged, four-way adjustable headrest. Each seat also features a sleek, integrated high resolution 9” monitor with USB power available at every seat. Standard power outlets to charge your laptop or tablet are also available at every seat, in direct view and easily accessible.