I don't really have an answer to jamesster's topic, but it does remind me of something I've noticed.
DYNAMIC ELEMENTS
Generally, games seem to be more fun to screw around in (in turn giving them better sandbox potential) the more "dynamic elements" they include. A dynamic element is what I call any part of a game that can be changed in a way that meaningfully affects other things. The most obvious dynamic element that most games use is a simple physics engine. Even before modern physics engines, games like Super Mario Bros 2. and Super Mario World had objects that could be picked up and thrown around, stacked up, and interacted with in interesting ways, such as mushroom blocks, springboards, and koopa shells; these are excellent dynamic elements because they're so fun to screw around with.
Many games build themselves entirely around one or two dynamic elements. Minecraft is based primarily around removing and placing blocks in the game world, and also includes at least two other dynamic elements in the forms of minecarts and redstone. Even mobs can be considered dynamic elements given their capacity to impact the gameworld itself. Garry's Mod is interesting because it was born out of Half-Life 2's most obvious dynamic element: its physics engine. Garry initially created it just because he wanted to rope things together. The Incredible Machine is a classic example of a game filled with dynamic elements, and it needs to be, because the entire game is about constructing Rube Goldberg machines to solve puzzles.
In fact, that's a good summary of exactly what a dynamic element is. If you can use it as a part in a Rube Goldberg machine (or any machine for that matter), it's probably a dynamic element.
