Koreatown for one?
I find myself walking across 32nd Street regularly when arriving at Penn Station from D.C. Always busy, lots of restaurants. But my experience with Korean food is limited to barbecue -- good for groups, but are there other options for the solo diner?
I am a Korean and I dine solo almost all the time. LOL
Try Gaonnuri. Food is in general a notch above the other K-Town places. (Just avoid cold noodles, or Naeng Myun in Korean, which is sub par)
They have some half portion appetizers (good to try various small dishes esp. when you dine solo), as well as lunch set menu/ course at relatively more reasonable prices than dinner. Its views are spectacular too.
Kathryn's suggestion of Cho Dang Gol is another good option. Pretty solid food there.
I go for lunch solo there sometimes and hit up Cho Dang Gol or BCD Tofu House for the soondobu. I actually enjoy it because they will still bring you the banchan and your table quickly fills up with a little banquet in your honor- like kosmose's 4th photo...A friend has also recommended
Arirang for noodle soups.
You could also go to Woorijip and buy some food. They have a hot table with prepared entrees and various refrigerated stuff, and they sell beer and soju too. I always see tons of people sitting at the counters there eating by themselves.
Gaonnuri has an awesome view, I'm glad they're still in business - I heard some ridiculous sky high figure for their lease, I have no idea how they make rent given what they charge for their food.