1. THE IMMORTAL CHARACTERS
No TV dramas can do characters the way period dramas can. And that's not surprising, because most of them are adapted from bona fide classics bursting with characters who've stood the test of time. Take Dickens. Nobody - literally nobody - was better at conjuring up delightful, loathsome, funny, garish, unforgettable characters. David Copperfield, his most biographical tale, gives us the ghastly Mr Creakle (the original demon headmaster) and the roly poly Mr Micawber. And it's not just about Dickens. How about adorable Matty in Cranford, or roguish Willoughby in Sense and Sensibility? All of them, unforgettable.
2. THE REVELATIONS OF LOST TIMES
One of the incidental pleasures of watching a period drama is how you discover stuff you otherwise would never, ever have known (unless your secret hobby is reading up on, say, Victorian labour practices, which if it is, you'll like these shows all the more). If it wasn't for the likes of Dickens and Austen, only specialist historians would have a working knowledge of the romantic rituals of yesteryear, or what it was like to be thrown in a 19th Century debtor's prison. You don't get these kinds of revelations watching a soap.
3. THE NOSTALGIA FACTOR
Anyone over a certain age will swoon while watching Call the Midwife, and not just because of how pretty some of the people are. No, that swooning sensation is actually all the nostalgia flooding your senses as you enter the bygone Britain of the 1950s and 60s: the music, the fashion, the vanished way people spoke to each other, the grub and grit and innocence and cosiness. And you don't even have to have been alive in the era in question to feel nostalgic - a good period drama will evoke a time and place so well, you'll feel like you know it as well as your own. Even if there were many, many more corsets back then.
4. THE LOVE STORIES
Five words: Elizabeth Bennet and Mr Darcy. Now that's surely a romance that trumps all others. But then, period dramas are rather good at serving up unforgettable love stories. Think of Sense and Sensibility, which doesn't just give us the turbulent, passionate, hopeless romance between sweet Marianne Dashwood and cad-with-a-heart-of-gold Willoughby. It also contrasts their burgeoning affections with the love adventures of her more sensible sister. Meanwhile, over in Cranford, we find ourselves desperately willing on the awkward love that blossoms between Dr Harrison and Sophy. Who'd have thought a spot of typhoid could be such an aphrodisiac?
5. THE INTRICATE INTRIGUES
The storylines of the best period dramas are as intricate, delicate and downright beautiful as dew-dropped webs. Whether you're watching the juicy private machinations in Pride and Prejudice, or getting immersed in the gossip of Cranford's good citizens, the various interactions between characters will lead to sudden twists and awakenings. Remember the angst over whether Wickham had really murdered his friend Denny in Death Comes to Pemberley? We still can't quite get over the shock of seeing him in the woods by the poor chap's bloodied body...
6. THE SHEER EPIC SWEEP
You know another thing period dramas have? Ambition. Sweep. Big themes, explored over huge, teeming canvases. Maybe the best example is David Copperfield, which follows our plucky young hero right from the start - the opening scenes show his crazy birth (well, the birth itself is fine, but the presence of Betsey Trotwood makes things very dotty indeed), and the narrative then takes us right through his turbulent childhood at an evil boarding school, right onto his first loves and attempts to become a writer. Meanwhile, Call the Midwife conjures up the whole culture of the country in the mid-point of the 20th Century, throwing us into the lives of dozens of East Londoners. And how about the community of Cranford being laid bare for our nosey pleasure?
7. THE CLOTHES
Yes, it sounds superficial, but come on. Who can resist a good bonnet? Not us.