Joss Whedon, the series creator, loves to toy with our brains. He likes to surprise us, to catch us off guard and turn our beliefs to dust. That's why we deeply hate him and love him at the same time. Here are some of the mastermind head games he's pulled on us.

Buffy and Angel

The masterpiece. Can anyone here truthfully claim that they didn't cry at least once because of these two. It too k me a long time to be able to watch becoming without shedding a tear, I must admit.
Buffy, the vampire Slayer, meets a young man named Angel. Undeniably attracted to each other, they slowly get closer and finally share a kiss when Buffy discovers he's actually a vampire.
He explains to her, that, yes, he's a vampire, but a good one, cursed with a soul. He no longer kills to feed, even less for pleasure, and fights evil too. Buffy is not sure she can fully trust him until he kills his sire, and former lover Darla, to protect her. They still decided to keep away from each other, because their love is impossible.
But they can't fight their feelings and end up seeing each other again. They officially become an item and have sex together, causing Angel to be in peace, to live a moment of pure happiness, free from his torturing soul, and therefore looses his soul.
Angel reverts in evil Angelus, the vampire he used to be and delights in torturing Buffy, attacking her family and friends and wrecking havoc on Sunnydale with SPike and Dru.
They plan on awakening Acathla, an ancient demon that will suck Earth into Hell, as Willow works on a spell that would restore Angel's soul. But it's too late, and Buffy is forced to kill him.
Several months later, Buffy is just starting to get over him, when her returns, with a soul. She hadn't actually killed him, she'd sent him to Hell where he suffered a hundred years of torture. She nurses him back to health and the get back together, but Angel finally breaks up with and leaves town, realising that she will never be happy with him.

Killing Buffy... twice

I think Buffy is like a cat. She has nine lives! The lead character wasn't killed off once, but twice!
The first time, an ancient prophecy leads Buffy down into the master's lair, where he bites her and leaves her to drown in a pool of water. Xander arrives in the nick of time and manages to revive her using CPR. She was clinically dead for a minute or so.
But then she goes and dies again, as she throws herself into a dimensional portal and is killed by mystical energy. This time she is well and truly dead - buried and all. Dead as a corpse rotting six feet under for several months. But since her death was not natural, Willow finds a spell that will bring her back to life, the same as she used to be. Technically...

Normal again

Is Buffy's life real or imaginary? Is she a super hero or a total psycho? The episode 'Normal Again' considers this question and was very well put together, dashing from the life we know: Scoobies, Dawn, vampires, etc. and another where Buffy is in a mental institution. In this second life, Buffy's therapist tells her that to get better, she has to kill off the other life, so Buffy prepares to kill her own friends in Sunnydale.
It puts a doubt in your mind, especially the final scene, which ends in Buffy's psycho life when the doctor says 'We've lost her'

The soul act, again

Oooh, let's curse the entire vampire population with a soul! lol