First of all, let me say that I am not an author. I have great respect for anyone with the discipline and dedication to sit down and write out their ideas into a full-length novel or even a full-length series. However, nobody's perfect, and there are a few common mistakes authors make that I want to address.
1. Letting a series go on past its expiration date
The culprits: Sookie Stackhouse, Anita Blake
An author should always know when it's time for a series to die. Once it gets past that point, plots become absurd, motivations are murky at best, and the author has to introduce ridiculous plot devices just for something new. When a series has crashed and burned, then crashed again somehow in Antarctica and died a slow death of frostbite, leaving behind a very nasty corpse, it's probably a sign that it should have stopped at that red light earlier. And I may have gone a little too far with that metaphor, but you get the point. Series should end when they end, not be reanimated only to wander around looking for brains to eat.
Success stories: Vampire Academy, Georgina Kincaid, Fever, pretty much any trilogy or stand alone
2. Lack of balance in worldbuilding
The culprits: Chicagoland Vampires, Harry Potter
Everyone knows the whole power equals responsibility thing. What a few people appear to have missed is that power comes at a price. The first law of thermodynamics applies just as much in the magical world as it does in ours: you don't get something from nothing. Whether the cost of magic is personal energy or sacrifice or tattooing your eyelids, it has to exist somehow. The same goes for supernatural creatures; they can't have great strengths without great weaknesses.
Success stories: Mercy Thompson, Women of the Otherworld, Vampire Academy, Wicked Lovely
3. Treating death as reversible
The culprits: Buffy, Night Huntress
Death is a real thing that shouldn't be cheapened by rampant resurrection. Bringing characters back from the dead is generally bad form (though there are a few cases I'll give you, so this isn't a totally clear issue). But on the whole, authors don't kill characters just because, and bringing them back makes their sacrifice much less meaningful. Death defines life. Without it, life is just an eternal existence (and read Tide Lords to see how that goes).
Success stories: The Hunger Games, Harry Potter
That's all for now! What do you hate to see in a book?