Having beat Nethack five times I can't say it's not a great game. That said, there are a lot better games, and even many better roguelikes. Dungeon Crawl Stone Soup is my recommendation. It's insanely difficult, but in a better way than Nethack (you get killed when you make tactical mistakes, not when you trip down stairways, get crushed by drawbridges, run into various insta-kill monsters, etc). Not that I want to discourage anyone from trying Nethack... it's just not the best introduction to the roguelike genre.
No, there's limited interaction by means of 'bones' files if you play on the same host.
If you die and certain conditions are met, you'll leave your corpse, all your items, and a ghost for another lucky (or unlucky, given that whatever killed you will still be around) adventurer.
There do exist some multiplayer roguelikes, Mangband
http://www.mangband.org/ is probably one of the most well-known/popular.