The story is linear; it has no branching. But to say there is no exploring is incorrect. There is an arsenal of hidden weapons and items scattered throughout the game that the player will miss completely if they simply follow the straightest path to the goal which is not very interesting.
There is a wide range of ways to play through the game as most who've been following the series are well aware of. Simply running to the goal may often result in the fastest time, but hardly the best play.
Go through with pure stealth and you can keep kills down to the absolute minimum, but will have fewer points to purchase new weapons and much fewer pick ups overall.
Use non-lethal kills on bosses and you'll receive some interesting bonuses, which I'm sure many have already figured out.
Not that anyone but the most hardcore MGS fan will do this, but probably the purest way to play these games are with only the Mk2 tranquilizer gun for regular play, minimum kills and heavy reliance upon the Octocamo without triggering any alerts on The Boss Extreme.
The ridiculously forked grading system based upon how you play through the game pretty much spells out what type of player you are based upon your style of play. There aren't any other games I can think of offhand that do that.







