Killing off Adam Monroe, the best villain they ever had.

Two Words:Season Three. Between Moe-Moe's David Cronenberg tribute, Sylar flip-flopping, almost total ignorance of the previous season, Hiro gets a rival, Hiro goes to Africa, Hiro loses his memory, Hiro thinks he's ten (WTF?), Hiro loses his power, Hiro gets rescued by a spectacularly silly Deus Ex Machina, and whatever the hell is going on with not-Niki. Not to mention character derailment, aborted arks, and bridge drops.

Peter's entire driving force in the latter half of season 2 was his desire to save Caitlin from the future. Then the third season just pretends that she never existed. Wonder what happens when someone is left in a future that will no longer exist?

saving DL after being shot only to die after being shot again becuase of his wife's third split personality that is never ever mentioned again or before and having his son be the only one to be even remotely upset about it

When they kept Ali Larter on as an actress but killed her character, only to give her a new one that had no connection to the plot whatsoever...

The brilliant plan at the end of Volume 4, forcing Sylar to become Nathan and lying to everyone about it, completely ignoring A) Claire's healing blood and B) this being the worst idea in the history of worst ideas.

The fact that there are so many idiot balls in circulation, no character can be said to have much actual development outside of his or her (current) idiot ball.

The inept fashion in which the show hides secrets from the audience for "dramatic" effect. The most ridiculous example comes in season 3 where Noah (I refuse to call him "HRG" out of principle because that whole thing was stupid for the same reason) and Claire are looking through a file of various villains. Noah describes three of their powers: one can manipulate metal, one is a living flamethrower, one EATS FEAR, and the other..."You don't want to know." The one she didn't want to know? SUPER YELLING POWERS. Might as well have used a bleep and a subtitle saying "The plot doesn't want you to know this yet, but we're inept at hiding it"

Sylar's Heel Face Revolving Door in Volume 3.

The fact Claire and Mr Bennet seem to repeat the same character arc every volume. Seriously it's always they're getting along, Claire starts to distrust him for whatever reason and rebels, usually doing something stupid, finds out he was doing it for her along or something similar and they reconcile just in time for the end of the volume. Wash rinse repeat.

The way Sylar has essentially moved from being a dangerous threat to becoming a lame combination of Marty Stu and Mega Man.

That the original premise was "what would happen if ordinary people suddenly got superpowers" and it has since mutated to "everyone was either manipulated from birth by the Petrellis, or is a Petrelli him/herself."

The fact that the show has a diverse set of interesting and powerful female characters, who die off at a rate so quick you barely have time to remember their names. At one time, you could determine which female characters would live and which would die off one simple facet of their personality... whether or not they had blond hair (it works until the very end of Season 3, with the notable exception of Angela Petrelli). Now, they've simply expanded to killing all of the women on the show, except the Cheerleader and Angela. Fatalities among prominent male characters can't even hold a candle to the fatality rate of female characters in the show. My favorite is the lady who had the ability to read objects and determine their past. Her lines? *horrible blood curdling screams*

Random Plot Incoherance. It doesn't help when Kring's Pandering To The Base backfires at a catastrophic level.

They confuse Evolution with The Force

The way how just as we have completely warmed up to a new (or new-ish) character, the writers go and kill them on us.

The way they kill any other interesting villain that is in the slightest way cooler than Sylar. Examples including Bob Bishop and Adam Monroe (Thompson and Knox can also apply).

Not killing Sylar at the end of Season 1 and turning him into a plot crutch.

Peter's arc after season 1. Season 1 Peter was a great character: young; reasonably accomplished but drifting, maybe even having a little quarter-life crisis; just enough force of personality to go to nursing school even over his father's objections, but not enough to escape his older brother's orbit; suddenly consumed by ideas he knows instinctively to be true but can't intellectually believe; trying to piece together his place in the world even though those closest to him are manipulating him for their own purposes... and there was no indication that he was anything less than reasonably smart; maybe even a little on the bright side.

For season 2 onwards, the only possible explanation for Peter's behavior is that he is a complete idiot.

Thanks for ruining him, writers!

Peter suddenly gaining Common Sense when he becomes powerless, but everyone else graduating from his Academy of Applied IdiotBalls.

The fact that ABSOLUTELY NOBODY will kill Sylar. I cannot count the number of times he's been in a position to be killed by someone with a motive, a willingness to kill, and an opportunity (hell, half the time he's already badly wounded or incapacitated) and they simply fail to finish the job. Most annoying case of Joker Immunity and/or Idiot Ball EVER. I'm starting to suspect he has another power and subconsciously projects "You don't want to kill me" to anyone around him.

Why, oh why, does Sylar's shapeshifting ability also replicate clothes that he's never worn (e.g. Nathan's suit) and/or contain metal (e.g. Nathan's flag-pin)? I shudder to think where he's stashing all those clothes!

All the wasted potential...

Elle's death :(

Whenever Hiro is abused as silly comic relief and made to act like a naive moron. There's a difference between idealism and stupidity!

I can't stand the deaf character. I don't know if they want to appeal to a new set of audience members, but I find her really annoying. Plus, she can see sounds s color? How the hell does that translate to composing masterful sonatas and destroying walls? I guess one of the writers must have read The Umbrella Academy or something.

What the hell did Mohinder have to gain by going to America and telling ONE GUY he was capable of great destructive power? Did Mohinder's idiot sense tell him that only he is capable of creating a plot?

When Claire became a whiny bitch. She got better, though, which is nice.

Why doesn't Peter absorb the abilities of the Haitian?

Tim Kring. That is all.

We were promised that with the beloved Pushing Daisies being canceled, Bryan Fuller would return to the show in the middle of Volume 4 and make it better again. His return was followed by some of the worst episodes in the show's history

How it's just ripping off "X-Men" wholesale now.

Tim Krieg's 'auteur stink' in honestly thinking (early in the show can be forgiven, but he kept thinking it even during Season 1's success) he could replace the whole cast of characters every season and that NBC would let him. EVERY single problem Heroes has had since the end of Season 1 can be traced to him having to rewrite all his plans because he didn't seem to grasp when the show became popular that NBC would want to keep certain characters. Whether it was arrogance, stupidity, wearing blinders, or whatnot, it's really the biggest problem.

How it's always been ripping off Supreme Power and Rising Stars, and dozens of other similar comics.

Oh, and lol racism Japanese people equals katanas

Matt Parkman and his permanently attached Idiot Ball. Nuff Said.

It can never, ever be as good as Lost. True Dat