iDoom46 said:
The closest thing to true non-linearity in gaming would probably be Mass Effect.
Can you expand a little on how ME represents the pinnacle of non-linearity, given that it conforms neatly to the old RPG trope of:
"Doom is coming, you must hurry to save the world! But, whilst you're out, can you pop down the shops and pick me up a snickers? and a healing potion? Oh, and the various misfits you've collected together to save the world have some issues from their past they'd like to waste your time...er...resolve."
Not that I'm massively knocking ME for being hideously linear or anything. I just think that it's a long way from being truly non-linear.
Back on topic:
Linearity is not inherently bad - indeed sometimes, it's awesome. I cite Metro 2033 as an example of when leading the player down the road to the ending and offering them little choices along the way that you barely notice creates a truly cinematic experience. Metro isn't a game with an awful lot of replay value, but it does a particularly good job of telling a compelling, atmospheric story without distracting you and itself from that story with a lot of fluff and padding intended to draw out its playtime by sending you off on sidequests.
Linearity is good when its employed as a tool that keeps you, the game and the story moving forwards to the climactic ending. FPS, by their nature fast-paced, benefit from tightly drawn, linear storylines that take you from the beginning to the end, revealing little bits of their plots along the way.
RPGs, on the other hand, tend to sprawl a little more and move slower than their shooter brethren, and in general, any levels-based system tends to acquire a large amount of non-vital XP and loot gathering quests in your quest to hit the games level cap.
Sandboxing is good when it provides extension to the game/depth to the world/fun activity and bad when it distracts you from how bad the metaplot you were meant to be following is.