I've been thinking a lot about this, and I think I have what would make a good rule to follow when submitting meme entries.
If the meme is just a pop culture reference that has internet-born derivatives, it probably shouldn't be submitted to the Meme databse.
We approved the meme entry for "That's what she said" but it was borderline. It wasn't born on the internet and didn't need the internet to spread. It has a strong history of use in pop culture prior to the internet. People who don't have computers will get that joke. It actually has a much stronger history of being an IRL meme than an Internet meme. But because there is a very huge, very widespread usage of this joke by the internet community at large we decided to research it and put it in the database.
But we have a recent request that I'm really leaning toward "no" on.
"You can't just X into Mordor."
It's a quote from The Lord of the Rings and it's creative in the sense that people substitute their own verbs for X and make funny images for it, but there is no back-story at all.
Laying out the meme entry would consist of the following:
1. Tell which movie of the Trilogy it occurred in, at exactly what hour and minute.
2. Embed a clip of the line.
3. Explain the very simple "substitute the verb in this sentence" formula.
4. Showcase the pictures.
But the reason for the meme's popularity is blatantly simple. It's just a reference to a wildly popular film trilogy.
If we were to accept the meme into the database, we would have to flood the database with Star Wars references. Don't get me wrong, I like me some Star Wars references, but Star Wars is not a meme. Just as Milhouse is not a meme. (However "Milhouse is not a meme" is most definitely a very strong 4Chan meme.)
I agree with an opinion that Sebastian has brought up on occasion. It can come from somewhere other than the internet, but it should NEED the internet to spread. This is why we are still researching "Ran Ran Ruuu." I would say that it is similar in nature to "Yatta." Both were mainstream media in Japan, but almost any time something from Japan spreads virally to the US and abroad via the internet and spawns any kind of parody it's worth researching.
I already know that a lot of users on this forum have expressed similar opinions. But if you disagree with this, or if you have any thoughts to share, let's here them.
PS: I'm having a lot of fun with these discussions. I don't think any other forum has had users who put such serious thought into analyzing both the memes, and the criteria under which they are accepted and classified. Beats the old Know Your Meme Wiki by a long shot.