French term (edited): Il voit immédiatement une montagne d'obstacles

To be slightly less effusive than 'a mountain of...', you might tone it down a bit to something like 'a whole heap of...'

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Note added at 40 minutes (2016-12-10 19:24:33 GMT)
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For a more natural-sounding word order in EN, you could try 'At once, they see...' — and this would have the possibly advantage of allowing you to run it directly on from the previous sentence, where the FR rhetorical question sist rather less comfortably in EN.

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Note added at 48 minutes (2016-12-10 19:33:06 GMT)
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I cannot subscribe to Barbara Cochran's claim that 'immediately' is over used — for a start, we have no idea if it ever occurs again in the source text! I certainly wouldn't want to see it on a recurrent basis, but I do feel that here, specifically, it does help to convey the—immediacy!—of the typical pessimist's reaction. I did, however, suggest 'at once' as being a slightly less formal way of expressing that immediacy; I'd be reluctant to depart much further from the regsiter of the source text.

I do think this is one of those cases where the adage "If it ain't broke, don't fix it!" applies — the source text is well written in a natural, non-literary style, and lends itself well to a fairly literal natural, idiomatic rendering in EN, avoiding the pitfall of adding the frills and furbelows of self-conscious style.

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Note added at 1 jour4 minutes (2016-12-11 18:49:07 GMT)
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To address a couple of issues raised:

The normally plural pronoun 'they' (et al.) is increasingly accepted these days as a 'neuter' singular pronoun, but as W/A rightly points out, needs to be used with an eye to the surrounding context; certainly, it CAN be used with a singular subject: "Ask a pessimist... and they'll tell you...". However, in certain of the circumstances we have here, it might well be necessary to make the subjects plural as well, for the sake of its "sounding right".

Regarding the use of 'mountain' — yes, I too had a hesitation here; that's why I was trying to look for an alternative, but as Nikki rightly says 'heaps of' is really too far out of register, as would be 'loads of' and any of the other similar expressions I was able to come up with.

I did also consider the idea of using an adjective to describe the obstacles, instead of a quantitative modifier; but I just have this niggling doubt that 'insurmountable' slightly over-translates here; though I hasten to add I don't have strong feelings about this! It's just that to me 'mountain' seems to me to suggest something that merely appears insurmountable to our poor pessismist. And I think there are some precedents for using a term like 'mountain' without its being hyperbole — "making a mountian out of a molehill" comes to mind, as well as Donna Summers's "Ain't no mountain high enough..."