There's this 'elegance' thing some people including me talk a lot about. But no-one ever explains exactly what this concept means, so here's an attempt.
I think it's something like, "explain what the card does informally. If shorter the description is, the more elegant the card."
For example, here are some elegant official cards:
- Mastermind: a delayed King's Court.
- Destrier: A lab that gets cheaper if you gain things.
- Cavalry: A card that salvages your turn if you need more cards.
- Smugglers: a powerful gainer that's restricted by what your opponent gained.
- Black Market: a card that can gain cards from outside the game.
- Golem: A card that searches and plays Action cards from your deck.
This isn't the same as amount of text (though it's certainly correlated). Golem is the classical example: as-is, it's wordy, but if 'dig' where a keyword, it could just say 'Dig for two action cards, play them in either order'. Similarly, Cavalry has a lot of text but the idea is simple. I've cheated a bit by not mentioning the upper half, but I think that's fair; I think of Cavalry as just being all about the bottom half. The top half is just some vanilla effect that happens to fit with the flavor. If it were +2$ instead (and the flavor supported that), I don't think I would bat an eye. It may make it less fun, but you probably wouldn't see it from looking at it.
This is why the attack on Scrying Pool is bad. Without it, it would be "a card that draws until it hits a non-Action card". Not bad. But now it's "a card that draws until it hits a non-action card and also Spy-attacks everyone". Why does it spy attack everyone? It's different from Cavalry because Cavalry needs some upper-half; Scrying pool didn't need the attack. It certainly doesn't need it for power level. (The self-spying is arguable, I think it shouldn't have that, either, but it's nowhere near as bad because it fits with the concept.) I think another example (one where I'm not just echoing what Donald said) is Goons. There's the concept, which is a card that gains points for gaining cards, and then it also attacks. It doesn't need to attack. The attack does nothing for the concept, and the card would be plenty strong if it didn't. This is different from something like Rabble, because Rabble very much is 'A smithy that discards good cards for your opponents'. The attack is part of the point.
If a card does have a short description, a related property is excitement. Basically, that's just 'how exciting is the description?' Again, I think all of cards above score well there. I also think that many people (but not all) evaluate fan cards primarily on elegance + excitement.
However, neither elegance nor excitement imply being fun to play. They're at best necessary; they're definitely not sufficient. I think smuggerls is wonderfully simple and sounds exciting, but I happen to hate the way it actually plays. Similarly for Goatherd, Masquerade, and Possession. Turns out I don't like things that punish you for doing good things with your deck. I wouldn't have known that without playing.
I honestly suspect that there are dozens of fan-made cards in the WDC alone that are every bit as good as official cards. (The format is just really good to find good ideas, as long as you look at the max rather than the mean.) I think if they had been released officially without having appeared here first, no-one would have batted an eye. However, I also suspect that there are many that look every bit as good but that most people would find completely unfun, and it's just very hard to know this from looking at them. For example, one of my favorite designs ever from the WDC is Touch of Midas by grep:
I find card is oh so elegant, and quite exciting. But if you told me that I would dislike playing with it, I would only be marginally surprised, because who knows. I think the same is true for lots of contest winners. I suspect that there is a mixture of fun and super unfun cards that won contests and in many cases, no-one can tell them apart. Here is where official cards should have an edge; there's a filter here that does something essentially invisible. It doesn't work perfectly, but there are very few official cards that everyone finds unfun. Some of the removed ones come closest; Tribute has a nice-sounding concept (give random vanilla boni based on opponent's deck) but it just doesn't work out, mostly because you get +4 Actions so often, and because it's too weak.
A corollary of this theory is that many official cards probably wouldn't win the WDC. Suppose Menagerie hadn't yet been released and someone submitted Destrier. Do you think it would win? My guess is no. It might get an honorable mention. But I think it would have a higher chance of being the 'best' card, as in, the card people would choose if everyone played a hundred games with every submitted card.