Fun
Fun is an intangible, but arguably very important element in Nethack. Death by boredom is a real risk if you're an obsessive player -- you get a little reckless, your judgement slips, and WHOOPS! WYLYPI?
Various types of scumming, and other game tactics which involve long repetition without actually breaking any rules, can increase your score or chance to win the game while exponentially increasing your boredom. These tactics include use of foocubi which potentially can increase your levels to a completely unnatural extent - filling the dungeon suddenly with much higher level monsters. Especially if you haven't encountered those monsters in your gameplay normally yet, this can cause a break in the natural progression of the game which involves the gradual introduction of new monsters.
Greed can be a big obstacle to fun, resulting in many YASDs (well, it's a risk, but if I use the jumping boots I could escape the shop with all this glorious booty!) Sometimes playing fair is more enjoyable. (On the other hand, sometimes the opposite is the case, especially if you have a really cool trick up your sleeve. Chaotic characters are often more fun to play for this reason).
Excessive reading of spoilers arguably decreases the sense of achievement of learning the game by yourself.
Playing with roles like the barbarian that are practically unbeatable in the early game may be less fun than playing a rogue, archeologist or a tourist, which require more strategy and care to play. Alternatively, you could have a policy of randomising your character role and race whenever you start a new game, attempting to get as far as you can with whatever the RNG throws at you. This can prevent you from getting in a rut, letting you get a taste of different styles of play. As they say, all games are ascendable.
Playing the game on a shared computer and competing for high scores is often fun. Sharing bones files in this manner is too. Bones files can be shared online using a program such as hearse http://www.argon.org/~roderick/hearse/hearse.html, which can be interesting and refreshing. (This one time I found a bones level in which THREE players had died before because of a centaur with a wand of sleep! And there was a magic lamp in one of the piles! That RNG is one fickle bitch.)
Writing up your ascensions on online groups is arguably fun, or at least strokes your ego.
YASDs are not fun. At all.
In Zen buddhist practise it is taught that you should do things for their own sake and live in the moment, rather than for any outcome. In this way it is better to place the emphasis on playing nethack one level at a time, without limiting your satisfaction to that elusive ascention. A game of Nethack can be an excellent spiritual practise for you. (If you reach enlightenment while playing Nethack, don't forget to credit me).
