Good game
Hey, great game you've got here. I know you don't think the walkthrough pop-up is annoying, but I think it insults my intelligence. Most of the time it was simply the timing of jumps and such that got me killed, it wasn't the planning or anything. It's a >puzzle< game, it's the point of a puzzle game to be puzzled! It's nice to have a walkthrough on the internet, but you don't need to link to it midgame, when most of us will just see it as a distraction to our purpose (getting timings right, etc.).
Some of the other features took me a while to get used to, and frustrated me at first: that when you touch a block on the side it will disappear, even if you want to land on it. I realize this is an important part of one of the levels, though.
Also, I agree with Keslen that the timer shouldn't start ticking immediately: you should wait until the player starts to move, or, taking that idea further, you could only count the time that the character is actually moving.
But overall, good game.