21. Undead Hordes Piling Up Around You - Left 4 Dead
ValveNo game captured the frenzied zombie mob quite like Left 4 Dead. In addition to procedural enemy spawn-points, Valves co-op classic featured 28-Days-Later-style undead that sprinted headlong and piled up dramatically as you messily mowed them down. It was honestly difficult to see sometimes, with all that meat in the air - you can probably chalk that one up to "End of the World Problems."
20. Breaking Time While Timing Jumps - Braid
As we fell deeper into Braid's painterly rabbit hole, its complex weaving of time manipulation and intricate platforming became simultaneously more dynamic and focused. Most levels had only one solution, requiring some intensely lucid exploitation of space, time and A Buttons. At a certain point, you'll move time forward by simply moving forward physically, which is as potent a metaphor as it is a brain-melting exercise in 2D puzzle solving.
19. Building Momentum From Portal To Portal - Portal
Portal's puzzles forced us to think outside the box, because there was no way we were getting out of that box without our Portal gun and a fierce commitment to non-linear problem solving. Falling through an interdimensional gateway we just shot from the hip was a far cry from Grandpa's jig-saw puzzles, but a welcome one.