Creator's Pet: Season two of the Australian version had ballet dancer Arron Smyth, who was pushed to the moon and if the judges and creators had their way he would have won it by a mile. The finale winner was chosen by viewers and he came second to this guy, Smokin Joe Robinson.
Sean Sheehan's "Chopping Wood" song for the first semi-final in the fourth series. Even Simon started singing along.
Likewise, DJ Talent; both Amanda and Simon buzzed him, but later regretted it and let him through after noting that it was actually quite good.
Ensemble Darkhorse: It's fair to state that Susan Boyle is one of the most memorable contestants in the show's history. Paul Pott, too.
Glurge: A pre-teen male soprano told everyone, after prompting from the judges, about being bullied at school, not having many friends and singing for his mum. The worst part was that he was talented and didn't need to say all this.
Ho Yay: David Walliams' attitude towards Simon Cowell. "My Simon" indeed.
Also a big reason why Ronan Parke didn't win Series 5, along with a blog entry by a supposed Syco employee who claimed that the show was rigged for him to win. The blogger later admitted that he made the whole thing up, but enough newspapers reported it as fact to cripple any chance Ronan had of winning.
Most Annoying Sound: The buzzers. Not only because they mean the act is one step closer to getting the boot, but because they are LOUD. A couple of contestants have remarked on how off-putting they are. They're so loud that when one contestant did a balancing act with sharp knifes, and Piers buzzed him in, it caused him to flinch and he almost didn't make it.
Narm: The over-the-top presentation during the semi-finals/finals can get a bit silly at times, generally during the video packages for each act, complete with Ominous Latin Chanting and fast editing for some very mundane acts (including flower-arranging in series 3).
Piers dressed up as Marilyn Monroe, singing "Happy Birthday, Mr. President", for a skit on the spin-off. What has been seen cannot be unseen.
Mike Hederson's audition. He was balancing on two knives with a third in the middle with the blade facing upward. Piers hit the buzzer as he was reaching for the knife. It looked like he just stabbed himself! All the judges was rightly terrified.
Rescued from the Scrappy Heap: Alesha was seen as a bad choice for a judge by some, but after a few years she has come into her own and is generally viewed as being a decent to good judge, especially when it comes to musical acts.