MGS3's controls shouldn't be taken for their face value. While some functions like peeking in first-person mode can have use on their own, there are many functions that you might miss that can come in very handy.
Opening Doors
Normally you can open doors by walking directly into them. Running into them at top speed will bash them open (which does not alert guards for some reason), while approaching them carefully will open them more slowly. You can also open a door partially and peek through - stand next to the door without opening it, enter First Person View, and use the 
and 
buttons to peek through the door.
Hanging
Flatten yourself against a flat railing (it must be about waist-level to Snake) and press 
. Snake will jump the railing and hang from the other side. Good for avoiding inquisitive patrols, but in the right context, also good for making difficult shots with the pistol. Beware the camo index, though—hanging from things like tree trunks will probably lower it to 5% or less. Veterans of Metal Gear Solid 2 will probably try doing pull-ups from the ledges (+) — unfortunately, you cannot increase your grip strength this time around.
Going into first person during a pull-up (++
) may give you a tactical advantage when you're unsure of the enemies path.
Grass Peeking
Crawling through grass will shift the game into first-person Intrusion mode, which means that 80% of your screen will probably be filled with grass. If you need to make a clear shot from the grass, though, hold , , and to lift yourself up from the grass just slightly to peek forward. Guards don't notice you any more than usual (unless your camo clashes with the grass).