What's your best score? Mine is currently 2.
Spelunky is great. Just completing it is a good challenge. A successful hell run is a great challenge, but if speed runs and ghost mining don't float your boat, what then?
For me, the best bit about a well designed roguelike is that every run might be a successful one, if only you play it well enough. Restarting for the perfect seed for a high score, or a fast time is less exciting than succeeding against a hard seed.
As a Nethack beginner I assumed that ascension required incredible luck. Player mrivan blew holes in that my managing the incredible feat 23 consecutive times. I would never ascended myself if it hadn't had my face rubbed in how fair the game was despite the random numbers. I just had to play better. People have since bettered mrivan's score, but his incredible run proved that it was viable to just play better.
I suspect Spelunky has the same quality. Almost every seed is hell-runnable, and those that aren't should at least be completable.
This is my attempt at encouraging the type of high level play that I find exciting.
Q: Why 0 points for regular completion?
A: Regular completion is much easier than a hell run. If you can score points from a regular win it may encourage long term streaks of cheap victories over hard, exciting games.
Q: Okay then, why does the streak continue after a regular victory?
A: So that you still have something to play for after botching the hell run. It also provides an interesting "maybe it's too risky for a hell run this time?" decision.
Q: Why do you lose the streak for skipping a day?
A: To prevent people from scumming by only running on easy daily challenge seeds.
Q: Surely any seed is a lot easier if you've already watched someone play it?
A: Yeah, this is a weakness. So, yeah, don't watch peoples daily challenge run before you play your own.