In terms of action games, I feel the classics did it really well by having hub-type (some would say maze-type) levels and hiding a ton of secrets within them. Y

In terms of action games, I feel the classics did it really well by having hub-type (some would say maze-type) levels and hiding a ton of secrets within them. You were actively encouraged to try to break the bounds of the map, and when you did, you'd often find little surprises, powerups, or even entire secret levels left there by the developers. I recently played Duke Nukem 3D for the first time in years and loved every minute of it. Cracking open a hidden wall and pulling out a weapon that shouldn't even be in that part of the game is so much fun. The last non-trivial secrets I remember in an action game* were in Serious Sam: The Second Encounter, where you could stumble onto random phone booths or warp into strange miniature worlds with a little exploration. Games these days are all about structured, linear experiences — a shame for a medium whose primary strength is, I think, learning and discovery. (As with all things, though, I expect we'll get a resurgence of "retro" FPS at some point or other. Really looking forward to the classic formula with modern tech!) * Actually, I just remembered that the Rise of the Triad remake had a lot of good secrets, including secret levels with RotT-classic looks. So there's another 21st century example for you!