Spelunky is one of the greatest video games ever created. It is pure perfection and I believe this to the fullest possible extent since playing the game for what must be seven years now.
It is an absolute treatise in good gameplay mechanics. Zero percent of it is an interactive story (save the intro and end, barely), it simply excels in being some of purest expression of what makes a game fun to play.
It has a little physics engine - one with no positional resolution (except for tiles) - that keeps the game constantly feeling smooth. The reason acceleration is put into a game, is because linear motion is so bizarre without it. That's kind of off topic but it's true.
What some people don't realize is that you can play spelunky however you want and it stays challenging, and the way you play the game changes over time. When I first started playing, I would methodically go through most of the rooms. Now I go straight to the exit. That may seem like hardly a difference, but the process was very much an evolution of giving up what I thought was necessary to beat the game using my own skills.
The only flaw I have found with Spelunky (and this is arguable): the teleporter is rng. mr yu pls fix.
I have so much more to say but I won't bore you.