Th main problem with "Snape kills Dumbledore" is that while it's probably the most relevant spoiler meme for our generation, the fact that it's modern also causes problems (i.e. people who are interested in reading/seeing Harry Potter, but spend enough time online to get hit by that major spoiler).
That said, I've read the Harry Potter series a number of times, and it's really not that much of a spoiler. You can tell from the beginning of that book that Dumbledore is going to die, and Rowling almost explicitly states it with the foreshadowing scene involving Snape and Draco's mom in the first few chapters. The series is still very good with that foreknowledge.
And, for the record, I've also found that it's a very common meme when people on the internet are talking about spoilers.
edit: oh, and Notch rules!