Unresolved plot threads.
Although my long-standing disdain for this particular phenomenon extends to pretty much any blatant instance of such abject laziness in overall basic storytelling technique, it's only recently been brought back to the fore and generally exacerbated by my having read Marvel's cosmic titles - from Annihilation through War of Kings - in their near entirety.
In all fairness, it's not my intention to single-out comics as being somehow guiltier of this practice (or even necessarily doing so more often) than anything else presented via other types of media. To that end, I present the following example (one of several I can think of off the top of my head - and bearing in mind that I haven't watched this show for over a decade) from Babylon 5:
The through-line of the series (except for an unplanned-for fifth season - one wherein they received a new budget from TNT after the series finale had already aired, only for the now-renewed show to be constantly rescheduled and barely ever shown thereafter) is mostly a build-up to a war against an evil race of aliens known simply as "the Shadows" - with said war ultimately playing out during season four. Anyway, it's eventually discovered that the planet shown in the background of every medium shot of the titular space station is actually home to an ancient device of incredible destructive power.
It's just the kind of thing which might turn the tide of that aforementioned war! In fact, two entire episodes are devoted to securing the weapon, and restoring it to its former fully-operational glory. So it comes as somewhat of a shock when the war is finally over, and the realization sets in with the viewer... that this so-called "super-weapon" was somehow completely forgotten, and ultimately remained unused!
The mere mention of unresolved plot threads from Marvel's post-Annihilation cosmic titles probably brings one thing to mind most readily for readers: Wraith. Here was a character that was introduced during Annihilation's sequel storyline Conquest - one who even got his own self-titled mini-series - and he was intended to then function as part war veteran, part folk hero to his people (the Kree) immediately following the story's resolution.
Now imagine what it would have been like if Lando Calrissian hadn't shown up in Return of the Jedi... and he was never mentioned again. Yeah - dropping Wraith was kind of like that.
But lest you think that I'm unfairly beating up solely on the architects of Marvel's recent cosmic series - Giffen, Abnett, Lanning, et. al. - I actually wanted to instead focus now on something that's been bugging me ever since I read Secret Invasion: a company-wide cross-over event, albeit one with admittedly cosmic (re: alien) origins.
All the signs from an ancient Skrull prophecy - a legend which tells of the alien race successfully conquering Earth (and that, in and of itself, begs an additional question: Just how long have the Skrulls known about Earth, anyhow?) begin to come to fruition. And then, whenever something goes wrong throughout the course of the actual invasion that follows, the Skrull Empress almost immediately makes a comment along the lines of, "This too was foretold!"
Now, putting aside for a moment the many contingency plans that the Skrulls had in place long before this invasion had even begun (almost as though they were still unsure about the final outcome) - and somehow also ignoring the fact that, even armed with the requisite foreknowledge, the Skrulls dutifully went ahead and made the same mistakes that they surely must have already known they were going to make - and once you've slogged through this entire series, you're left with the following sad conclusion:
So the Skrulls' prophecy is seemingly one-hundred percent accurate - that is, up until the exact point where it turns out that [SPOILERS!]the Skrulls still lose.[/end SPOILERS!] And naturally, Brian Michael Bendis offers up no explanation as to why the prophecy upon which this invasion was so clearly predicated somehow manages to get the most important part wrong.