Japan's national museums aren't that large; I can think of only one real national museum in the country. Korea, on the other hand, really piles the artifacts on at each museum. As you've seen, the Agricultural Museum, Police Museum, and War Memorial and Museum all have significant collections and detailed information about their subjects of focus.
The National Museum of Korea is an art history museum, and it, too, is simply massive. There are several floors in the gigantic building, each with several exhibition galleries.
The exhibits are arranged generally by time period, with similar objects grouped together in most cases. There are so many objects that a true art lover could easily spend a whole day or more here!
My interest lies in realistic, intricate works - usually, beautiful painted ceramics, statues and sculptures, and paintings of unique landscapes or scenes.
However, I also enjoy seeing the art and beauty of science, industry, and society, so old writing, furniture and architecture, maps, and official seals also catch my eye.
Three years ago, I had very little interest in Asian art. However, living in a country that creates it and visiting several others has brought me a greater appreciation for this genre. With that also comes a better (though not good) understanding of Eastern religions and, thus, enjoyment in religious works.
It looks like the National Museum has a small collection of two-dimensional works, though there is a greater percentage than pictured. I like the styles of art for paintings you see here. But the collection of three-dimensional materials is outstanding!
There is a large cafeteria serving local, Asian, and Western dishes that was crowded but good.
The museum opens daily (Tuesday through Sunday) at 9:00, closing at 18:00 Tuesday/Thursday/Friday, 21:00 Wednesday/Saturday, and 19:00 Sundays/holidays. Admission is free, though for 3000 won (~$3) you can rent a PDA or 1000 won you can rent an MP3 audio guide to really enhance your visit. There is a limit to the number of people who can visit the museum at the same time, though there are no tickets required. Special exhibitions might have an admission charge.
Take subway Line 4 or Jungang Line to Inchon Station and use Exit 2. There is an underpass from the subway to the museum called "Moving Museum" that was a nice addition.