For TV writers, for example, it's become a go-to twist to drop in at the end of an episode. And in shows that are built on twists, this becomes an issue when they've pulled this one out of the bag every six episodes and by the fourth season everybody is related. We're not exaggerating, by the way -- just look at Lost. Claire and Jack turn out to be half-siblings. Daniel Faraday is also half-siblings with Penny. That creepy Horace guy turns out to be creepy Ethan's dad. The only two Asian characters introduced after the first season turn out to be father and son.
Thankfully they dropped their planned "they're brothers and brothers" reveal.
In fact, one of the biggest shocks of the series comes when John Locke discovers that a man who he thought was his dad was just trying to scam him out of a kidney -- they had so many surprise relatives in this universe that the twists came from the fact that two characters weren't related. Heroes fell into this pattern, too -- Hayden Panettiere's character is Nathan Petrelli's daughter, then Meredith Gordon's daughter, then Flint's niece (and if you don't know who any of these people are because you stopped watching after the first season, that's a good sign).
The last season involved an evil super-carnival ... seriously.
It's bad storytelling for the same reason the Star Wars prequels were bad (well, one of the reasons): Each new connection makes the universe of the show or movie smaller. Finding out that Darth Vader built C-3PO makes it seem like everything in that vast universe revolves around a few families and friends. It's the same in any story where it's abused -- characters who meet in a cool chance encounter turn out to be long-lost brothers or some shit, because this rich fictional landscape is actually made up of just one very unlucky family.