One minor nit: if the board is full, but you can make a move that will free up a space, you can make that move and the new tile will appear in the newly opened space, but then the game immediately ends afterward. EDIT: OK, apparently this only happens if there are actually no more moves; as long as moves remain the game continues.

Also, hitting "space" reset the entire game; I'd expected it to either do nothing or add a tile without moving.