You can stop now, game! We're sufficiently impressed!
It seems that the stumbling block for when deciding how much content is appropriate is the developer's own confidence. Much like the content itself, the problem seems to be that there's either too much of it, or not enough. An all-too-confident developer won't quite understand when the game has outstayed its welcome, whilst the meek will litter their game with collectibles and dull secret-searching – an almost apologist gesture.
There's a lost art here, one born of subtlety and nuance. The common argument is that value can only really be measured in player enjoyment. And whilst some of us have known this for years now, many developers haven't quite caught on. The theory seems to be that our enjoyment is measured in how many things the game gives us to do; one need only look at the bloated mass that is
Resident Evil 6, or even the less interesting side quests in the otherwise excellent
Arkham games, to see this ethic in action.
The skilled developer doesn't bullet point the reasons their game deserves to be played endlessly. It skilfully hides them, rewarding those who crave, ask and search for more. Don't give us fifty different skills or stages; give us five, and have us work them to exhaustion. After all, it's the player who ultimately decides whether or not it's worth it.
And I mean, seriously.
How freakin' long was Metal Arms?
LOOK WHO CAME: