I think it is acceptable, nay mandatory, for RLs to kill you quite quickly the first time. But ideally, even the most inept player should see something interesting before he dies. Getting some sense of beginning (talking to some people, getting a quest, travelling in an overworld, whatever), and then being killed by a frog after two minutes, is better than just seeing a little @ move around when you press the keys, and suddenly getting a message that "a stone block fell and crushed your head."
When it comes to mid- and end-game difficulty/low winnability I think RLs are generally very hard, and I wouldn't mind some more lenient ones. Imagine a RL where an intermediate player could get a lucky win. Getting killed by random stuff is IMHO part of what makes the genre great, so I think a 100% winnable RL is an absurd notion. My taste says there should be situations where you just die to bad luck, but also ones where you slap your head afterwards, or better yet -- heave a proud sight of relief after a narrow escape, where you had to rely on your wits and your luck, and prevailed!
Then there's this argument that random content ensures replayability. But that should also mean that the game is just as fun after you won it. It demands that you implement an endgame that is random and fluid just like the midgame, instead of one that revolves around (getting certain powers to face) certain, fixed levels/bosses/obstacles. But if you have 5 locations, 5 overlords, and 5 fatal weaknesses to kill the overlord, you can get 5^3 variations on the final showdown
Computer games in general are often about clearing levels to unlock content. In some RLs, it can feel a bit cheap and needy, like the game is trying to get me hooked with promises of cool content deeper down in the dungeon, but allowing me very slow progress. Compare to board games, for example, where you get an overview of the entire game after a few sessions, but it stays enjoyable because the pieces of rules/content continue to come together in new, astonishing patterns.
I think I would find very sympathetic, a roguelike that let me win after a hundred games, but that I might still pick up to play, because losing is almost just as enjoyable as winning.
As always,
Minotauros