It's strange but I am finding myself kinda missing the Mako in ME2 (probably because I find the scanning mechanic a little... irritating at times, especially for a completionist like me ;-) ).

But back to the question in hand, for me I like to take screenshots when I see something that makes me go "Wow!". So for example I took a bunch of screenshots the first time I entered Forochel in LotRO and saw the night-lights in the sky there - very cool moment for me.

Incidentally, for story heavy games (or ones with definite milestones), I would love an automatic screenshot system similar to that in Dragon Age: Origins where the game would take the screenshot for you when, for example, you beat a raid boss for the first time, or when you complete a long and complex quest line.