I've learned to expect game environments that feel substantial and relevant. Ironically, it was Valve that reset the environmental interactivity standard with Half-Life 2. Half-Life 2's remarkable physics gameplay, coupled with the brilliant inclusion of the gravity gun, changed my expectations completely. When I push, I want my game world to push back. By Half-Life 2's standard, including the near-rudimentary Half-Life 2 deathmatch, the worlds of Left 4 Dead and Team Fortress 2 feel comparatively lifeless.
Of course, there's far more to interactivity than cover-based combat and physics simulations. Consider Ubisoft's recent Prince of Persia titles, to name one example, in which the Prince literally grapples with the environment at every turn. He shimmies along ledges, slides down curtains, swings around stalactites, and generally grabs, climbs, and wall-runs his way through huge environmental puzzles. You can't play your way through a Prince of Persia title without feeling a powerful connection to its polygons.
The same is true, to an even greater extent, of the remarkable Shadow of the Colossus. Clambering across the massive bodies of the titular Colossi, traversing their spines, holding fast to their fur, and climbing their limbs, is a singular, terrifyingly intimate experience. The Colossi would be no less remarkable if you attacked them from a distance with ranged weaponry, but they'd be far less believable and real.
Even simple, seemingly inconsequential things make a difference. I don't know if anyone else does this, but almost every time I walk into an in-game bathroom for the first time, I check to see if I can turn on a sink or flush a toilet. I look for my character's reflection in the mirror before I see if it will break. I'm compelled, as I suspect most players are, to test the limits of the illusion at every turn. Every little interactive feature adds yet another layer of believability.
Bugs on the windshield. Wheel ruts in the track. Grasses parting as you walk through a field. Shadows across the sword in your hands. Wet clothing and footprints as you emerge from a pool. Handles to grab. Towers to topple. Blocks to push. Crates and barrels to smash. How real a game feels often depends upon these sorts of ingredients and how they're incorporated into the environment.
To be fair, Left 4 Dead does appear to include a decent helping of these features, though they're primarily cosmetic. If my friends' lists and the forums I frequent are any indication, for many gamers it's a near-perfect package. In my case, it's just not quite enough. I need games I can grab, wrap my arms around, and smash my shoulder up against. Anything less seems artificial.
Adam LaMosca is a writer and researcher in Portland, Oregon. His hobbies include flushing toilets and turning on sinks.