Whenever new sets are being spoiled I see a ton of people point to a big, dorky new creature and say, "That card has to be great in Commander!" In a lot of cases, it's not true.
There are a number of different categories that the best cards in Commander fit into, and a few characteristics you can look for in new cards to figure out if people may want them. This also applies to older cards that haven't found the right home yet.
Of course there are factors that affect the prices of all cards, like supply. But there are other important ones to consider when looking to make Commander specs. Let's take a look at the biggest factors that influence Commander play.
I think a lot of popular consensus is that Commander is filled with decks playing cards that all cost five-plus mana. The reality is most decks and most popular cards are really cheap powerful ones. According to EDHREC.com, of the top 21 cards played in Commander, 20 of them cost three or less. The sole card that costs more than three is Sun Titan which is an obviously powerful and flexible card in any deck.
The reality of the situation is that high-mana-cost, powerful cards can't be the bulk of any deck, and cheap, efficient cards are more likely to be expensive. For example, it's a lot of the reason that Anguished Unmaking is so much more popular than Scour from Existence. In general, a new card has to be more efficient than an existing popular card to get played.
Efficient cards are great but if they're only good on turns one-five and not turn 30, they're less desirable to players that expect to get to six-plus mana every game.
It's hard to judge just how much flexibility matters because it varies from person to person, but generally speaking there are some pretty clear-cut winners. Cyclonic Rift is the most played colored spell in Commander because it's great with two mana and almost oppressively powerful at seven.
My best example of how drastically flexibility can impact a card is the popularity of Cultivate compared the popularity of Nissa's Pilgrimage. There are about 30,000 decklists that include Cultivate and about 3,600 decklists that include Nissa's Pilgrimage. The mana fixing is a big deal and the inflexibility of Pilgrimage costs it a ton of play.
This is probably the most obvious part of this list. Every time a card says "each opponent" instead of "target opponent," it scores more points in this category.
Hour of Devastation has a lot of cards that affect all opponents and will be climbing the ranks of EDHREC as time goes on. Nicol Bolas, God-Pharaoh has two abilities that affect all opponents and Torment of Hailfire is already one of the most expensive rares in the set, despite seeing no competitive play.
Cyclonic Rift is another prime example that you will see appear on this list because it affects all of your opponents when you overload it.
Power is subjective and the most likely factor to be influenced by efficiency. Sol Ring is the kind of card that will be included because the number of games you win on raw card power from playing this is too high.
On the flip side, cards like Craterhoof Behemoth, Deadeye Navigator, and Insurrection are not cards that are extremely efficient but they're overwhelmingly powerful. Usually when you cast one of these spells you are close to winning the game on the spot. Sometimes these are cards that gain an overwhelming advantage over a few turns, like Consecrated Sphinx, Elspeth, Sun's Champion, or any of the titans.
A trap that a lot of people fall into is seeing new powerful cards and not figuring out where they would appear in a deck. Even if there were 15 different cards that behaved similarly to Craterhoof Behemoth, you can't play 15 cards that cost eight in your deck. Something would have to be replaced. If you can't figure out a card that is much worse than a newer card it's unlikely players will adopt it.
If there's one thing I've learned about less competitive players, it's this: when they find something they like, the only thing they like more is doing it twice. The price of Doubling Season, Primal Vigor, and, more recently, Anointed Procession and Panharmonicon, is a small tribute to how much players love to double stuff.
It doesn't matter what it is either—just double it. Want to gain double life? Pick up Rhox Faithmender. Want to gain double mana? Look no further than Mana Reflection. Want to draw double the number of cards? Pick up a Thought Reflection. What I'm saying is, historically, there is a huge demand for cards that double your stuff. Within that niche, there is definitely a preference on doubling counters and tokens.
Synergy is hard to describe or quantify, but I'll try my best here. Synergistic cards are generally not particularly powerful in a vacuum but help to further your plan.
For example, Contagion Engine is a synergy card seen in decks that play a lot with counters. Atraxa, Praetors' Voice is the most popular Commander on EDHREC, and Contagion Engine is one of three cards in the deck's top 10 most popular cards that isn't part of the precon. Understandably, this card has seen a huge jump in price from $4 to $10 after her release.
Outside of counter-based strategies, Contagion Engine isn't played and isn't as popular. The reason that Contagion Engine is popular and expensive is because it has great synergy with Atraxa (among other Commanders).
A final and crucial piece of information that can affect a card is the uniqueness of its effect. Is the effect already present in its color? Is it similar to existing cards but with a twist?
Most recently, Anointed Procession is a card that's more unique than it looks at first glance. It's basically a colorshifted version of Parallel Lives, a $10 rare from Innistrad (released almost six years ago). A lot of token Commander decks are green and white, and would probably play both of these effects. However, there are also non-green decks that play white and make tokens.
Anointed Procession can go into Temmet, Vizier of Naktamun, Queen Marchesa, Breya, Etherium Shaper, Daxos the Returned, Darien, King of Kjeldor, etc. I missed all of this when Anointed Procession was spoiled. But if you are able to identify it early you can already see that casual demand can cause a Standard-legal rare to be the most expensive card in the set!
Be careful with what you decide is unique enough to be interesting. Unesh, Criosphinx Sovereign is very unique but there isn't a lot of support for Sphinx tribal outside of him, which makes him a bulk rare for now. Maybe in the future if we see a lot more sphinxes he will get better.
I wouldn't expect to make a ton of money quickly by investing in him now, but if you are able to acquire him for bulk pricing I can't imagine selling them to a store. I personally think that Sphinxes are too iconic not to eventually get enough support for a deck.
Commander players don't go all-out to buy powerful cards. They will often settle for less expensive but similar cards because they are less expensive. You can see from the amount of people who play Demonic Tutor compared to the people who play Diabolic Tutor.
More people play Demonic because it's not terribly expensive and is much better—however, there are a lot of decklists that don't include it and do include Diabolic Tutor because, for most intents and purposes, they're the same card in their eyes.
There isn't a hard and fast rule on this because everyone's budget is different. But it's a lot of the reason why most green and black decks don't play Bayou, most do play Overgrown Tomb, and almost all play Command Tower.
Picking the Perfect Spec
While from time to time you will find cards that hit one or two of these categories, I can't recommend going too deep unless it's excelling in many. Cyclonic Rift is basically the perfect Commander card as it hits almost every category except doubling. If we ever have another card like that, it will be sure to be expensive.