[Question] Unique game mechanics -- The best and the worst

As far as the best and worst unique gameplay mechanics go, I have to give both titles to Trespasser. Not only was it the first (or at least, first to use it in a widespread way) use of physics in a game, it's also to date still the only game that uses physics-based animations. This means that physics was not restricted to ragdolls and static objects: virtually everything that moved in the game had physics applied to it, from the dinosaurs to your own arm and gun. If you ran straight into a wall, your gun would run into it and get pushed back, even causing you to drop it if it twisted too far. Enemies too could take advantage of this: if a raptor gets too close to you, it could push your gun to the side to keep you from shooting straight at it when it's right up against you. Enemies could also trip over objects in their environment, rather than simply moving up and down on an invisible plane when they have to "step" over something like virtually every other game does. It was truly a remarkable and still unique gameplay mechanic that made you rethink how to play the game.

The bad comes from the fact that the feature was horribly broken. It was amazing to see barrels smashing into dinosaurs and watching their body react smoothly to the hit as they recover, or having your gun ripped from your hands by a raptor as it plows into your arm, but it was extremely rare to see the system work like it was intended. Dinosaurs (even the T-rex) would get stuck on the TINIEST objects in the environment, it wasn't unusual to see a giant reptile literally have it's foot locked under a 6 inch rock unable to move, just sorta leaning forward like it's trying to get out from under the inescapable obsticle. Boxes never seemed to work properly: for as fancy as the movement controls for held objects were, they always seemed to have a mind of their own, refusing to properly stack or stay where you put them even though there's no practical reason they should move. And sometimes, the physics just seemed to completely bug out, either getting your hand pinched on an object and seeing it stretch across vast distances or finding doors that never wanted to open even when you pushed your hand against it. Sometimes it was almost as entertaining watching the game screw up as it was watching it work right.

So anyway, that's the most notable example I could think of at the moment. Definitely a case of a game being too far ahead of it's time (it was released on the same day as Half-life, back in 1998) and lacking the polish it needed. If a game like that could've been running on the Far Cry engine and had a animation-based physics engine that worked as well as the upcoming Euphoria looks to, it could've revolutionized the entire genre (and probably several others as well). As it stands, we had to wait an extra decade before someone figured out how to get the idea right.

Jam it back in, in the dark.