Fracture
It's easy for gimmicky shooters to fall into the trap of the one-trick pony, where depressingly trite FPS tradition is given a band-aid of slowing down time or turning into a monster or some crap like that. What Fracture does right, and brilliantly at that, is actually incorporate its gimmick into every aspect of the game world and gunplay. Fracture's claim to fame is environment deformation, and your commodity tools for this - raising and lowering the ground, and destroying structural supports - can be used for everything from creating cover, to removing enemy cover, to reaching new areas, to absolutely hilarious killing blows. It's not just an element for environmental puzzles, and it's not just an element for changing up the basic combat: it's both, and it's actually quite fun. Of course, nailing the basic requirements of a shooter (aiming, grenades, etc) helps, too.
Progress: Finished the demo