I put little stock in seatguru anymore. It seems for awhile now that it's just people complaining that their seat wasn't a throne gilded in gold with the whole plane length and width for room. The 737-800 is the same F class type seat that you'd get on any US airline. Only difference is the DL uses leather while some airlines use fabric. (I prefer fabric since fake leather makes one sweat)

The really only bad DL F seat is the Domestic 767 which has a width of only about 1 or 2" more then Y on the plane. It's only as wide as Y is on most aircraft. The not great, but not really bad is the 737-900. I try and avoid the 900 as much as possible, and refuse to fly a domestic 767.