Some preview cards are a mystery wrapped in a riddle inside an enigma.
Some preview cards suggest an unusual way to change the game that is hard to understand.
Some preview cards leave you scratching your head wondering how to use it, where to use it, and if it will be good...
This is not one of those preview cards.
Thassa's Rebuff is the most straightforward preview card I have ever spoiled. Here is the one-line article:
"It's a counterspell for Thassa decks."
How do you use it?
When your opponent casts a spell you want to counter, you tap two lands and put Thassa's Rebuff on the stack. A good rule of thumb is to count their untapped lands and make sure your devotion to blue is greater than that.
Where do you use it?
Well, the card's name is "Thassa's Rebuff," so I'll give you three guesses, and the first two don't count.
If your Constructed deck has Thassa in it, you should consider her Rebuff. If your deck does not have Thassa in it, it is unlikely you will want this.
Will this card be good?
Yeah, it is good enough to play, but it doesn't exactly change things all that much. Thassa decks often have Judge's Familiar and Cloudfin Raptor, so it is very realistic to have permission up on turn two. It isn't as good late-game as Dissolve, but it is generally going to be Counterspell often enough to make it worth saving the mana. Getting to play a Thassa's Rebuff on turn five to protect your Thassa is a fine play, for instance.
I don't think Thassa's Rebuff is so great that you are suddenly going to play a lot more permission in Thassa decks than you would have otherwise. It is also not so good that you absolutely have to play all Thassa's Rebuffs and no Dissolves.
I would start by swapping out the Dissolves for Rebuffs and seeing how it goes. The real question is going to be how the rest of the format changes. Two counterspells being the "right" number in the previous format doesn't mean it will be true for the next one. Additionally, having access to a two-mana one makes it more likely you want more than two counterspells. Dissolve costing three leads to diminishing returns much faster than a two-mana counter.
Cards like Claustrophobia, Cyclonic Rift, and Rapid Hybridization all need to be compared to Thassa's Rebuff. I could see wanting to play three or four Rebuffs, but given how much you can play around Thassa's Rebuff in the right circumstances, I can also imagine wanting to play two Thassa's Rebuff and one Dissolve or perhaps a fourth permission spell that is either of the two.
Why is Thassa's Rebuff good?
Well, it is just a better Spell Siphon, but that really isn't the highest bar ever conceived.
It is kind of like Spell Rupture, which is similarly powerful but more fringe. Thassa's Rebuff is better positioned because it operates on exactly the same axis that Thassa pushes you and plays nice with all the cards we already want to play. The effect it provides is exactly what we are in the market for. So while the card is hardly revolutionary and isn't going to break anything, it does give Thassa and those devoted to blue a dangerous new weapon that will be used.
More exciting are the implications of such a card getting printed. Mono-Blue Devotion is already a tier one dominating force in Constructed. Giving it a two-mana counterspell is a powerful tool, but it does increase the ways people can try to play against it.
Additionally, Wizards of the Coast is obviously not scared of Thassa and her crew. Maybe they are just a bunch of reckless thrill-seekers, but I think it's more likely they have just pushed enough new cards in alternate directions that they believe at least another couple of archetypes are coming to break up the unholy trinity of Thassa-U/W/x-Black.
The absolute best thing Wizards of the Coast could do for this set is give enough support for two more major archetypes, possible chances for fringe decks, some tools to fight the big three, and a little bit of juice to help the big three.
Why help the big three at all?
Those are the three most popular strategies in the format at the moment. A whole lot of people are playing them. How boring is it when the strategy you've been playing gets nothing from the new set? It is pretty lame to have to play no new cards.
Printing cards that offer new tools to dominate existing decks without breaking things is a very real challenge. Thassa's Rebuff in this light is actually genius. This is not just a minor twist. Costing two has very real implications and will change the way the format is played. Thassa's Rebuff is something people want but might also be so effective that some number of people go back to Dissolve (since you can play around Thassa's Rebuff).
Thassa's Rebuff is super simple on the surface, but how many do you play? How does having permission a turn earlier change things? We are talking about the difference between Mana Leak and Cancel. That is a really big change to the texture of games.
Theros radically changed the game, and Born of the Gods is about to take the format on another sharp turn. Theros made monocolored decks a real big thing, and Born of the Gods is about to pull it back toward two-color decks. Thassa's Rebuff is a holdout to the old monocolored world, but who knows? Maybe, just maybe Thassa's Rebuff is good enough for two-color devotion decks using the new goods to be interested. Maybe, just maybe Thassa is tricking us. After all, a two-mana counterspell is no joke.
Will Thassa's Rebuff be good?
Yes, and it will help one of the best archetypes, but hopefully it won't flood the format with counterspells in a way that takes control of Magic.
Printing it might have just been a stroke of genius...