Watching the trailer for Skyline, I think I've figured out why so many alien invasion movies annoy me so much:

Seriously, why are these gigantic aliens coming here? For an alien-invasion movie to be plausible, there has to be a reason that the life forms or society that's invading have decided that Earth is worth the trouble of resistance, the trip, the set-up efforts of colonization. With aliens this large, it makes very little sense. If they're coming to settle, those things are huge, and aren't going to be supported in any great number by our planet's resources for any amount of time that would make this worth the effort. If they're chomping up planets as they go, we'd probably have noticed something. If they have some sort of religious belief about annihilating other life in the galaxy, that would be interesting to know! If they're just mistaken, or off-course, or screwed-up in some way, they probably wouldn't be wreaking such havoc and we'd be watching District 9 instead, which actually sounds pretty good as an alternative. Answering these questions isn't dreadfully hard. Doctor Who has figured out a rationale for alien arrival pretty much every other episode for about 800 episodes, which even accounting for mailing it in, suggests it can be done. So even if it takes ten minutes to pick something random, writers and directors should pick it, and stick to it. If only for the sake of looking like they tried.