Coincidentally I too have just played it, without any introduction to the game (must have missed the tigfront post back then). After finishing it I ran here with a wall of text forming in my head, ready to post, but after reading this topic I guess I'll just sum up.
I generally like the feel of being dropped right in the middle of a story. What kept me playing was the feeling that I will return to the boss after I reach some goal in the water part. The ending surprised me and, well, made me want to share my reflections with you guys. This is a design success!
I'm also not sure about how different my experience would've been if I fell into this first pit. I didn't, so I won't speculate.
Oh, and on a second run I encountered a bug in the first part, the character dissappeared after being hit by one of those falling blocks but I was able to go back to the beginning of the stage, where he fell from above, like when you start the game.
Ace music!