Plausibility! The byword of stories and alternate history timelines alike, a necessity if you want people to follow along with the tale you're stringing; people have to be able to accept that what you're telling them could be the case, that you're *not* just leading them down the garden path. Good stories need to accord with the world as we know it to be true, or they're fatally compromised as a result.
Sometimes it's difficult to really wrap one's head about why this is important. Recently I came across an article on Gizmodo regarding the Pentagon's withdrawal of support from the movie The Avengers. As author Spencer Ackerman put it, their reason was that "the Defense Department didn't think a movie about superheroes, Norse Gods and intergalactic invasions was sufficiently realistic in its treatment of military bureaucracy." Presumably, the implied conclusion we're supposed to draw is that this is ridiculous, hair-splitting stuff, and that the Pentagon is just being a bunch of jerks who want to cramp the movie's style.
You know what, though? The military is right. According to the Defense Department, their main problem is that they couldn't figure out where the US military stood in relation to S.H.I.E.L.D., which Wikipedia describes as an "espionage and secret military law-enforcement agency," which really narrows it down - and, hell, I imagine it's easy as hell to maintain secrecy over something like a giant flying aircraft carrier. S.H.I.E.L.D. has, from what I understand, been the subject of fan debates over just what it is for a good chunk of the last fifty years.
Answering questions like this is important. They define what you can and cannot do in a story, and as such reduce the unmanageability of everything being possible into more restricted channels that can guide the flow of a narrative. Something that is shadowy, nebulous, and ill-defined even to the people writing it does not lend itself well to the best writing. Creators need to know how their creations work, even if that information never filters down to the audience.
While you could in theory make a plausible case for, say, this public art installation being in fact part of a point-defense laser system, unless you've already established a world where things like that are known to happen it's not going to fly very far.
Here, I speak from experience - while I was still planning the background of some "future investigation" stories, one of the biggest hurdles was establishing the organizational background they'd lean on so that I could have a better idea of what sort of stories I could plausibly write. Real-world agencies were right out of the picture; unless you're actually a part of something like that, or have invested a huge effort in studying it, it's extremely difficult to write one accurately. While the idea of a general "world police" was tempting, given the potential involved in globetrotting adventures, it's not very plausible - how many countries do you think would be willing and eager to hand over their law enforcement to, say, the United Nations? Would any?
Here's the simple, unalloyed truth about The Avengers - the presence of superheroes, Norse gods, and intergalactic invaders acts as an add-on to the world as we know it, and does not give creators license to freely abandon the world as we know it unless they show us how that presence changed it. It only comes up as a strange-looking issue because most people don't know how military bureaucracy works to begin with. If it was something more familiar to the common person - like if cars suddenly ran on molasses instead of gasoline with no reasoning or explanation - the point, I think, would be a lot clearer.
There's no problem in bringing new things to this ordinary world - just so long as you're doing it, you don't contradict the way we already know this ordinary world to function.