As an internet meme college dropout and failed PHD of bullshit and jokes, after ruminating on this for a moment I came up with this additional condition. Let me know what you think:
When talking about jokes, the setup is often based on some form of template or specific subject matter (X Y and Z walk into N or "What's the difference between X and Y", jokes about professions, race, gender etc.), and the punchline is always different. You know the setup, or at least its outline, but not the punchline.
When talking about memes, the setup is different every time, but the punchline is always the same (Arrow in the knee, the above example of "it was me, Dio", picture-based memes like Philosoraptor and Conspiracy Keanu). You know the punchline, but not the setup The funniness, like in all humor, comes from the subversion of the reader's expectations and how well the punchline fits into the setup. A meme is only as funny as long as it can be applied to new contexts without feeling overused.