Spoilers for both Harry Potter and Buffy the Vampire Slayer under the cut.
Buffy and Harry Potter were in fandom cahoots when both emerged in 1997. It's not hard to see why; both series had similar themes (friends as family and love being a driving force, for instance), both had Power Trios (with very similar dynamics), and I think there's just something about both series that appeals to people..
Of course, there are also differences, and in the 17 years of fandom (yeah, I'm old school), I've observed criticisms of Harry Potter I felt were often actually reflected in Buffy. And I've wondered if that's really fair; on one hand, it's certainly worth arguing that there are things JK Rowling might have done differently, with Buffy as an example. On the other hand, I think it's important to recognize that even with similar themes and characters, the creators were ultimately still building very different universes, and writing very different characters.
Let's talk about the most obvious: Draco. Fandom Draco is dry, sarcastic, and hot. He probably looks like James Marsters with blonde hair. He wears leather. Wait - that's Spike.
Certainly in fandom, Draco has taken a Spike-like role. He is often paired with a "good guy" - initially it was usually Hermione/Draco, then fandom quickly shifted to Harry/Draco as a more popular Draco pairing. Ginny/Draco was another pairing. The romance was often played for tension as much as anything else, particularly with Harry/Draco. The trouble is that some fans seemed to want canon Draco to have a similar story.
But the character Spike is closest to is probably Snape, not Draco. Snape isn't hot like James Marsters, but he teeters on the edge of redemption while still often screwing the characters over until the very end. We never quite know where his loyalty is going to fall. He had a stalkerish love for Lily, the way Spike initially had an unhealthy crush on Buffy. He's the one who delivers the snarky lines.
Meanwhile, Draco could be seen as a composite of three characters: Cordelia and Andrew and Jonathan collectively. Like Cordelia in season one and early season two, Draco is a bully who insults the trio on a regular basis. He believes it is his right because he was born into a wealthy, pure family. Slowly both Cordelia and Draco begin to realize that they don't necessarily want to continue the paths they've been made to follow; for Cordelia, it's on a much simpler level - she's in love with Xander, who is less popular than she is. For Draco, it's when he's told to kill Professor Dumbledore. Even then, Draco doesn't entirely cut off his friends or family; he simply refuses to do it until Snape steps in, and he also refuses to identify the trio when they're captured in Deathly Hallows. Draco doesn't fully abandon his evil ways, he just realizes there are things he's unwilling to do.
Two other characters Draco is similar to are Andrew and Jonathan. On one hand, Andrew and Jonathan were always outcasts, and this is ultimately their motive: no longer being geeks, but badasses. Draco is quite popular, if with Slytherins more so than anyone else. Still, he has a similar mindset: he'll have power if he joins Voldemort. He'll be badass. Like Andrew and Jonathan, even after he still does bad things, but he also wants to be a better person. He never wanted anyone to die; he just wanted to feel powerful, and got too deep into it before considering if that was truly the kind of person he wanted to be.
The difference? JK Rowling didn't want Draco to be a hot, snarky love interest. Fandom made him one. JK Rowling saw Draco as more of an overprivileged bully who was taken advantage of by a ruthless leader and convinced to do pretty terrible things. That's it. (Although Andrew does get a romance arc with Anya.) So it's not really fair to compare Draco and Spike when both characters were very different.
The other difference is how JK Rowling sees magic, and this is reflected in Hermione and Willow. Specifically, let's talk about Marietta's Facial Scars of Doom and Willow's magic addiction. On one hand, JK Rowling didn't have Hermione take things to quite the level Willow did at times, but she did have Hermione curse a classmate with permanent facial marks - and seems to think this was okay, because "I loathe a traitor." I honestly have issues with this reasoning, but that's beside the point. Willow, meanwhile, also goes too far with her magic (if MUCH further than Hermione ever did), but this is portrayed as dangerous even before season six. (Can I just say "bored now" to the whole magic addiction plotline?)
Of course, in Buffyverse, magic isn't natural. If anything, we learn that special gifts are demonic in nature. When Buffy is able to hear people's thoughts, she begins to go insane because humans aren't meant to have that gift. Even Buffy's Slayer abilities alone are the result of a young teenage girl having been captured and metaphorically raped by demons so she could slay vampires. This isn't to say magic is always bad - after all, not even all Buffyverse demons are bad. But magic is something mortal humans don't naturally possess. When they do come to possess it, they're supposed to use it responsibly. Often the characters do the opposite. In Harry Potter, proper use of magic has to be learned, but it's also something one is born with. Muggles aren't born with any; Squbs are born to magical parents, but don't have magical powers. In any case, magic can't be exploited any more than it can in Buffy, but it's existence isn't a matter of borrowing and using responsibly; it's a matter of not being a bad person. Voldemort and the Death Eaters use magic because they're awful, not because magic made them awful. So Marietta's punishment is presented as less horrifying; perhaps to JK Rowling, it's more like Hermione had punched her for it, and that left a scar. (In Hermione's defense, the Ministry had gone 1984, and Marietta could've gotten everyone into a lot of trouble.)
Then there is the tonal difference, which we see in Sunnydale and Hogwarts.. Sunnydale made for the worst high school experience ever, in part because it was directly over a Hellmouth. When Sunnydale High is destroyed in season three, this isn't a tragedy, but part of a victory. Same with season seven.
Hogwarts, meanwhile, is home for Harry. It's a place of hope and friendship and family. There is still evil, but evil must be fought to keep Hogwarts safe. Hogwarts having been destroyed forever would not have been a victory for Harry, but a horrible loss.
And that's an important difference in the endings of both series. Buffy is definitely about love and hope, but it's also about rebuilding. It's more cynical in this way; we leave Buffy and the Scoobies surrounded by rubble, yet Buffy still smiles. We leave the trio waving their kids goodbye as they depart for Hogwarts. One is the ending of an action film, the other the ending of a fairy tale. But as dark as the books got, a fairy tale of sorts was the story JK Rowling was telling. Hogwarts was a princess in its own castle, and Harry saved it so they could all live happily ever after. Sunnydale High was an American high school where even without vampires and other evil things, there were snooty bullies and horrible principals (although if the first one hadn't been devoured by hyena people...). It was a bad place that wasn't even fully redeemed when it was rebuilt. The Hellmouth wouldn't allow it.
In short, I love both series, even if they have various flaws, and even if there are some things I think one series did a little better. But as similar as they are, they are, in the end, quite different - and I don't think it's fair to criticize one against the other.