The controls have been tightened up. Weird glitches in interactions between different objects sometimes have been smoothed out. There are more audio and visual cues about things that are important in various levels. Some of the levels have been redesigned to work better — though the redesigns are usually concerned with details of the level geometry, and the basic ideas behind the puzzles themselves are still the same.

The sequence of worlds that the player visits, and the behavior of time in each of them, has not changed. (Though some minor details have — for example, what happens when you die while immune to rewind?)

Basically, the changes are all focused on taking what was there and making it as good as we can; and not around fundamentally changing the game or adding huge amounts of things, because we feel that the core game is solid, and it doesn’t need anything added.