That's a pretty good score! Glad you enjoyed it. And thanks for the feedback, I actually want to talk about a few of your points.
• Make powerups stand out more. They are currently too dull colored.
• Add two big arrows pointing down the shaft. They can disappear after a while to not clutter the screen.
That's more than fair. I changed the flicker speed and time on the powerup pieces to make it stand out a bit more, without becoming annoying. I also added an audio cue when they spawn. The arrows idea is really cool, I'll be implementing that soon.
• Power up idea: Temporarily widen shafts.
• Power up idea: Increased gravity. This would help shapes slide down faster.
• Instead of a fixed height for spawning pieces they should drop from above the current highest piece [...]
• Add lives? One shape spilling [...]
Those are good suggestions, in fact I had something similar to the first two written down for a later version/revision, one in which you will be able to tweak all of the options to make it the exact type of challenge you want to play. For instance, now you can only configure the speed and difficulty, but later on you'll also be able to change parameters like lives, powerups that change the play area (like the ones you named), as well as powerdowns, for instance one that does the exact opposite of widening shafts and leaves only one shaft open.
• Add an "About" button in the menu for brief overview of gameplay.
I actually pondered this quite a bit before and decided against it after considering how one would approach the game. Take for instance some of the old classics, like Tetris or Puzzle Bubble (not that I dare compare my game to any of them, I'm talking purely from the player's point of view): you get your hands on a game cartridge, on a console or a portable device, you download a ripoff or clone from some website or get some '500 games in one' bootleg cds. Or maybe you buy one of those weird LCD battery games. Chances are you're not getting the manual, and even if you do you don't care enough to read it most of the times. You want to play the game, so you often play it 'wrong' for years before recognizing what all the things do, in other words you create your own play-style, rather than doing what the manual tells you to.
I will include some instructions, it will be in an attached readme file (plus there will be online info explaining the rules), but I somehow don't wanna take away from that raw gameplay approach. Anyway, rant over