What exactly was so special about the Bat Bogey hex that Ginny simply being able to cast it made her good enough for the Slug Club? Was it really hard to cast? Is there any reason why anyone couldn't cast it?
However, when a person’s bogies turn into black bats, which crawl out of their nostrils and flap away you can generally count on them shutting up long enough for you to talk for a change." —Miranda Goshawk
I don't think it is an ordinary spell but neither is it as powerful as the Unforgivable Curses. We see through out the series that school students have trouble transfiguring thing. Now imagine turning bogies turn into black bats. It can't be very easy. It looks like a wizard has to be powerful for it to work perfectly.
For example Harry being able to cast corporeal Patronus when he was in school marks him out to be powerful wizard.
“You produced a fully-fledged Patronus?”
“Yes,” said Harry, “because -”
“A corporeal Patronus?”
“A - what?” said Harry.
“Your Patronus had a clearly defined form? I mean to say, it was more than vapour or smoke?”
“Yes,” said Harry, feeling both impatient and slightly desperate, “it’s a stag, it’s always a stag.”
“Always?” boomed Madam Bones. “You have produced a Patronus before now?”
“Yes,” said Harry, “I’ve been doing it for over a year.”
“And you are fifteen years old?”
“Yes, and -”
“You learned this at school?”
“Yes, Professor Lupin taught me in my third year, because of the -”
“Impressive,” said Madam Bones, staring down at him, “a true Patronus at his age... very impressive indeed.”
Harry Potter and the Order of the Phoenix, Chapter 8, The Hearing
I think the same applies here. It's how well Ginny can perform this spell that marks her out as a powerful witch and impresses people like George Weasley, Fred Weasley, Ron Weasley, Hermione Granger, Horace Slughorn and others. There are some quotes which indicate this.
“A weapons going to be a lot bigger than the Stone, though!” said Ron.
“Not necessarily” said Fred.
“Yeah, size is no guarantee of power,” said George. “Look at Ginny.”
“What d’you mean?” said Harry.
“You’ve never been on the receiving end of one of her Bat-Bogey Hexes, have you?”
Harry Potter and the Order of the Phoenix, Chapter 6, The Noble and Most Ancient House of Black
Couple of Stunners, a Disarming Charm, Neville brought off a really nice little Impediment Jinx,” said Ron airily, now handing back Hermione’s wand, too. “But Ginny was best, she got Malfoy - Bat Bogey Hex - it was superb, his whole face was covered in the great flapping things. Anyway, we saw you out of the window heading into the Forest and followed. What’ve you done with Umbridge?”
Harry Potter and the Order of the Phoenix, Chapter 33, Fight and Flight
“Oh dear!” chuckled Slughorn comfortably, looking around at Ginny, who was glaring at Zabini around Slughorn’s great belly. “You want to be careful, Blaise! I saw this young lady perform the most marvelous Bat-Bogey Hex as I was passing her carriage! I wouldn’t cross her!”
Harry Potter and the Half-Blood Prince, Chapter 7, The Slug Club
“ENOUGH!” bellowed Harry, who had seen Ginny glowering in Ron’s direction and, remembering her reputation as an accomplished caster of the Bat-Bogey Hex, soared over to intervene before things got out of hand. “Peakes, go and pack up the Bludgers. Demelza, pull yourself together, you played really well today, Ron...” he waited until the rest of the team were out of earshot before saying it, “you’re my best mate, but carry on treating the rest of them like this and I’m going to kick you off the team.”
Harry Potter and the Order of the Phoenix, Chapter 14, Felix Felicis