I'd like to suggest a compromise:
CardA + CardB + [category] is a reasonable thing to post as a neat and potentially useful card interaction.
For example, Tunnel + Apprentice + [sifter] gets lots of free gold that Apprentice can use to draw large hands. Of course, that could be further generalized to Apprentice + [free gold] is good, but you get the point. There are enough sifters in the game to make a combination like that relevant. Likewise, categories like [+buy] or even [draw] are reasonable; there are certainly good two-card combinations that aren't good unless there is a third piece to the puzzle.
But even that compromise suggests a bigger-picture issue if you're thinking about posting a multi-card interaction.
In another thread, someone posted a video of a Dan Books turn 6 win as a result of something like a 4-card combination (Storeroom, Talisman, Villa, Peddler) He used Storeroom to put multiple Talismans together, did a buy-one-get-three Villa, Repeat until empty accumulating ridiculous buys, buy out the Peddler pile, then buy out a third cheap pile for a win (on Wolf den points, but could probably have pulled off a win on a couple of Estates instead.)
Posting that 4-card combination is useless, but the individual combinations are worth noting. But even then, the more general synergies are probably more important to get in your head than the specific combinations. Not Villa + Talisman, but Villa + [gainer], Peddler + buy, Talisman + buy, Talisman + sifting, Storeroom + special treasure, etc.
As a general rule, I would suggest to anyone thinking about posting in this thread, ask yourself this question: Is it really neat? As in, is there something unique that that two-card combination does? Could you not substitute another card for one of the others and achieve exactly the same result?