Raw nagaimo, or "long potato," is a starchy tuber similar to African yams, and is appreciated in Japan for its neba-neba qualities. There's no fair translation for this onomatopoeia, but it refers to a magical kind of slippery stickiness... if there were a nice-sounding word for slimy, it would be neba-neba.
In the US, such foods are often treated with suspicion, but it wouldn't be fair to dismiss this texture outright; Japanese cuisine is more about experiencing contrasting textural experiences than, say, complex seasoning or elaborate technique.
Other neba-neba foods include cooked okra and nattō, and, to a lesser extent, the sea vegetable mozuku. I will never be as big a fan of nattō as Hiromi is, but that's thanks more to the aroma than the texture. I love okra, especially cooked with onions and tomatoes. And mozuku is a favorite treat of mine, served as a simple side dish with a chilled, almost soupy, lightly acidic dressing.
Nagaimo is a kind of mountain potato, or yamaimo. If you grate it with a daikon-oroshi grater, you'll get a madly viscous mass called tororo-imo which can be mixed with a raw quail egg, simply seasoned with soy sauce and chopped scallions, and poured over rice at breakfast. Tororo-imo is also indispensible for making good okonomiyaki.
Fresh nagaimo also makes a nice side dish when cut into matchstick slivers (sengiri), as seen above. This brings out the neba-neba qualities while retaining a pleasantly crisp texture. I now typically use a mandoline to make this task easier; however, in a pinch, a good chef's knife will do. Just expect the cutting board—and your hands—to get slippery. You can avoid that by wearing latex gloves while preparing the dish. You may want to wear gloves while peeling the skin anyway, since some people suffer from a mild itchiness on skin contact with yamaimo skin... I'm lucky enough not to have that problem.
Once cut, place the nagaimo in small serving bowls and splash on a little soy sauce. For the flavor garnish, sometimes I add some chopped umeboshi and kizami-nori, or thin strips of nori. This time I used chopped scallions and a wasabi-seasoned nori, cut into strips with kitchen shears. The goal is to have a little saltiness, a little crunchiness, and some clean but sharp contrasting flavor. This version would be called sengiri nagaimo to wasabi-nori.
For an even more sticky experience, the nagaimo could be mixed with mekabu (wakame sprouts)... but that would be a lot of neba-neba for one night...