The player needs to be introduced to many mechanics: movement, respawning, corpse-block incrementation & limits, bouncing, ground types & pushing, water, lasers, repulsion beams, stacking, switches... but what order is it best to feed them to the player?
We've been thinking about this question a bit lately. Here's a pic of the board we've been making notes on. We've divided the current swathe of puzzle drafts by the hardest mechanic each contains, then ordered them within each category. Each mechanic will need one or two 'introductory' levels to demonstrate the mechanic and teach the player some techniques it can be used for. Past that, the actual puzzles (i.e. levels where the player is challenged in some way) have been arranged by rough difficulty. The introductory levels for each mechanic should go in the order shown, but the remaining puzzles should be interleaved for better variety: it's much more fun to play a variety of puzzles than it is to do eight or nine laser puzzles in a row.
Water is a tricky one. It's a very simple mechanic, but it could be introduced before, between, or after lasers and repulsion beams. Certain puzzles for both latter mechanics require water, but many do not. Similarly, we've pencilled in a seventh category—arrangement—that isn't actually centred on a mechanic at all, but on puzzles where the challenge is getting access to the newest (green) block. These are puzzles like Matt's wonderful BlockSwap draft, which requires the player to push blocks in and out of the same laser, so they can place the green block in a space where it's needed two deaths later.
There's a long way to go yet – even after working out the order of mechanics, the introductory and educational levels, and all the other puzzles we want to put in the game, there's still the matter of the difficulty curve, complexity variance to avoid fatigue, and mixing concepts so it never feels 'samey'. More updates to follow!