From a movie:
Do you think you could bring this back for me? It's not the easiest thing to fly with.

Why superlative? Why not simply something like 'It's not easy to fly with this thing'?
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I'd say it stresses the fact that it's NOT easy. There are quite a lot of easier things to do than that.
I think it's also a way to express something politely; for example someone tells you a story and you tell him: 'well, it's not the most interesting story I've ever heard' and the intention may even by that it's the least interesting; but probably you wouldn't say 'it's the most boring story...'.
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Quite right, Maverick!
It's a shortened form of the idiomatic phrase "be the easiest thing in the world" (be extremely easy).

Just my two cents.

If it were a kind of idiomatic phrase, or 'fixed' phrase, such a nice expansion as marveric88 has made would be impossible.

If there are thousands more of other expansions of this kind of superlative usage, I'd say it's more than an idiom.
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Visit [url="http://www.freesearch.co.uk/dictionary/the+easiest+thing+in+the+world"] here[/url]
OED quotes 8 sentences using "the easiest thing" and five of them are followed by "in the world".

Hello Taka

If the task were the 'easiest thing (in the world to do)', it would be no trouble to perform.

In this instance, it's not the easiest thing (in the world to do), i.e. it will cause some trouble.

So it's an understatement: the implication is, 'there's no way this particular task could be mistaken for the easiest thing (in the world to do)'.

I buy the idea of understatement. That seems plausible. Thank you.

If you like a dictionary-kind of definition, paco, idioms are those which 'cannot be understood from the individual meanings of its elements', according to the American Heritage; I have no trouble understandeing 'the easiest thing in the world' without using a dictionary.
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