So I was one of those people who had neither read nor watched Harry Potter (unless you count my fourth grade teacher reading the first book to the class). This year, I felt finally that I wanted to be able to have an opinion on the series. So I watched the first six movies. And I realized I was only getting part of the story. So I read all the books in three weeks (quite a feat considering it was in the middle of the spring semester). Then I watched the movies again, finishing with the seventh. That enabled me, two months later, to actually see the final one in theatres.
So here is my case for the Darth Vader and Severus Snape comparison.
1) Storyline: George Lucas said that he had envisioned the original series as Darth Vader's story. This is not unlike how many readers consider the book series Snape's story, more so than Harry's.
2) Background: We are first introduced to both characters in a "present" time, before we launch backwards to learn about their real motives.
3) Love: Enter those real motives. Vader/Anakin, of course, loved Padme, and Snape loved Lily. Both characters could not ever really move past these first, early loves. In fact, both relationships took place at about the same time--ages ten-ish on. The tragic difference is that while Padme did choose Anakin, Lily did not choose Snape. Which leads us to . . .
4) Power: As a Slytherin, we know Snape has interest in power. Anakin has serious issues with power and control, which ultimately cause him to be taken in by the Emperor; he essentially sacrifices Padme in his very attempt to keep her safe. After this, it seems like he loses his will to maintain any goodness in himself. Now Snape is a little different. His interest in power also led to his interest in a dark force, Voldemort, but at an earlier stage than for Anakin. Snape's choice to follow Voldemort was, it seems, what led Lily not to choose him. So when Snape's actions lead to Lily's death, he, too, is in that awful position of despair. Only he takes quicker action than Vader does.
5) Overthrowing the Evil Guy: It turns out that Darth Vader's character arc really is kind of cool: in the end, he is convinced by his son's unwavering belief in the Good to turn from the life he has been leading and finally overthrow the Emperor. One does wish he could have had this glorious revelation, say, twenty years earlier, but, hey, better late than never. Snape, however, turned to Dumbledore as soon as he knew Lily was in danger, which put him on the "good side" from much earlier on. I will admit that you can argue that this is exactly what Anakin did when he listened to the Emperor's words and did as he instructed just to keep Padme safe, but, no, I still think that Anakin was more unstable than Snape. Neither one has the best concept of Good/Evil, it is true, but Anakin's a little more scatterbrained and less focused than Snape, I think.
6) Visually: We can't leave out the fact that both these characters wander around in black robes/capes that billow out behind them. Both make imposing figures, in their own way. And you might just be able to say that the literal mask Vader wears is akin to the figurative mask Snape wears when he goes about his day as a double agent.
7) The Others: Let's not forget to match up the characters that surround them. I've already said that Padme is to Lily and the Emperor is to Voldemort. You can add in Obi Wan to Dumbledore, as well. And don't forget Luke to Harry--although Harry is not actually Snape's son, there is just as much of a connection there (if not more) as between Luke and Anakin. James Potter gets left out entirely, unless you say that he shares traits with Obi Wan (think Anakin's jealousy of him). The trio of Luke, Leia, and Han Solo is not entirely different from that of Harry, Hermione, and Ron--at least you can say that both trios include the hero and the two buddies who end up "together." Let's see, then there's Jabba the Hut to Mundungus Fletcher and R2D2 to Dobby and Chewbacca to Lavender Brown . . . maybe I should stop now, eh?