I was bored so I downloaded the original free version the other week and I finally just beat it.
It was overall a fun game but I don't think the design lived up to its potential. When I heard it described as roguelike-like and first started playing, I imagined going through the game and accumulating those power-up items, learning about the game's layout, etc., and then executing a skilled no-death run from start to finish to beat it. But I quickly realized that the most efficient way to beat the game was not to become actually good at it but rather to exploit the shortcut guy by repeatedly running each "world," giving X gold to the tunnel guy, dying, rinse and repeat.
Eventually I unlocked the last shortcut and then it was just a matter of figuring out how to beat the final boss. Initially I thought maybe despite having a shortcut to the last world I would need to do a full-game run anyway because it seemed to me that the final boss was very difficult to beat without any items. The boss himself is no threat but he's constantly summoning annoying monsters to attack you. However, while exploring the final world, I decided to kill one of those caster mummies to see what he dropped and lo and behold! He drops the most OP item in the game, an auto-aim insta-kill infinite-ammo infinite-range gun. The very next game I go to the boss and easily win. Zzzzz.
I also think the outcomes of your encounters with enemies, especially in the later levels, are too binary. In the first few levels, there's some attrition. If you make a mistake, you take damage but you can still continue and you can look forward to finding that girl to regain health. From the second world on, you can no longer survive most mistakes and if you do it's still sort of pointless to continue because you've lost so much life. Case in point are those Yetis in the third world and those Bloodseeker-looking (Dota reference) berserker dudes in the fourth world. If you're unlucky enough to not be pixel perfect when you try to stomp on their heads or get past them, they grab you and launch you across the screen, which 95% of the time means instant death. In the third world, being tossed means you fall down into the abyss. In the fourth world, if you get tossed you usually bounce off a wall and immediately get tossed again with no opportunity to break the combo. And what purpose does it serve to make the player sit through a 2-second stun before regaining control of the character? Sure, it makes the game harder but it doesn't make it more interesting. Part of the charm of real roguelikes is that even if you get yourself into big trouble you can sometimes save your ass with luck and skill. In Spelunky, there is no saving yourself once you err.
If Spelunky were designed better, the shortcut guy wouldn't exist and, more importantly, no one would miss him.