I firmly believe LA Noire, was a showcase experiment for R* to test out certain aspects and then using them to fuel their future games.
The revolutionary facial animation which was the USP of the entire product, along with the interrogation system, which was pretty suprising and well-implemented, and also the setting itself.
The above USPs work well in any setting, but I guess choosing the 40s might be the decision that was fuelled purely by the choice of popular games released in following years - Godfather 2, Mafia 2 and that's about it.
As you've pointed out correctly, there was absolutely nothing else in the game that stands out.
Story was mediocre, the open world is worse than Mafia 2, and NPCs are not upto R* standards, And too many open threads which could've been easily their own subplots.
But I guess that was the point of the experiment. They threw multiple things at once and waited for what actually sticks and then went to implement it in future games, and we got GTAV and RDR2 - pinnacle of modern gaming, both in terms of Facial animations, Storyline and the setting.
Game did suprisingly well during release and even won BAFTA. So all in all, a pretty successful experiment.
Thanks for sharing your thoughts through this article.