These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Paddy and Nico, a sweet, grandmotherly 79-year-old woman and her 39-year-old dance instructor. It begins as a Moment of Heartwarming: when asked if she wants to win, Paddy says that it'd be nice, "but as long as people enjoy what we do..." After that, the Moment of Awesome happens when she reveals that she has the athleticism of a 20-year-old and dances as such. Their performance is so good that Amanda uses her Golden Buzzer. They later end up taking third in the competition.
Designated Hero - Nobody knows why Piers Morgan, a disgraced ex-tabloid editor who insists he should not apologise for printing faked photos that endangered the lives of British soldiers in Iraq, is a judge on this show. His presence is even weirder on the American show seeing how he is known in the U.S. for absolutely nothing other than being a judge on America's Got Talent (and now for replacing Larry King).
To a lesser extent this applies to Amanda Holden and Kelly Brook. They've both demonstrated some degree of skill in acting and dancing, respectively, but neither have exactly set the world on fire in these fields.
Ear Worm - 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.
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.
And then the tabloids reported that the so-called "bullying" was actually (gasp!) a lie intended to provoke sympathy. The public turned on him after this was revealed. Once again it bears repeating that this boy was actually an incredibly talented singer and didn'tneedto lie, but probably did so after some subtle hints from the directors of the show. It's worth noting that in an interview, the judges flat-out stated that the more Glurge a contestant's tragic backstory has, the more likely that they'll pass the auditions regardless of their actual level of talent.
Basically every season has, "THE Glurge act", even if the act is very talented, and often makes it far. Plenty of the finalists/semi-finalists have the sappy backstory (doing it for their kids, etc) but it's only mentioned in passing, and it's not really that important. There is always one (TWO in season 2) that is completely based on their backstory (every video segment mentions it, they never shut up about it, etc). For most, though, it backfires when they are not voted into the finals.
A general Britain's Got Talent rule of thumb is that the more Glurge one acts brings with them, the further they're likely to get, as with its American counterpart.
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.
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).
Nightmare Fuel - 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.
Squick - The aforementioned Shotacon, from the female fanbase. As well as the occasional act that involves self-mutilation (see: Nick Hell.)