First, I will rank the abilities in Theros.
1. Scry: card advantage is great. Scry gives you the ability to filter your deck. Scry 1 is limited, but Scry 2 and greater seems really good. Scry stands on its own-i.e. the card with the ability is playable by itself. It is well represented in low CMC commons and uncommons.
2. Heroic: It requires an enabler, and in some cases gives you a benefit only once. It is well represented in low CMC commons and uncommons.
3. Devotion: It comes on its own, and you get it just from playing your permanents. It is found only sparingly, though.
4. Bestow: its a fatty ability, what else can I say?
5. Monstrosity: its a fatty ability that requires lots of mana, what else can I say? Sure, if you pull it off, you are likely to win, but if you are one mana short, you are one mana short.
Now the colors:
1. Blue. It has the most scry cards, and a really good selection of low CMC counter spells, as well as your usual flyers.
2. Red. It has some scry cards, lots of really good removal, heroic enablers, and low CMC heroic cards.
3. Green. One word: mana. Also, it has a great suite of creatures, only most of them don't fly and are vulnerable to blue counter spells and red spot removal.
4. White. Some good heroic enablers and heroic creatures, and your usual weenies.
5. Black. This is really sad, because black is my favorite color to play in Magic. Black has the Theros lottery card-a.k.a. Thoughtseize, the lowest cost bestow creature, some good devotion cards, and even some cool removal. But cool is not good enough, and neither is a lottery ticket. Scry favors blue, heroic favors Naya (white, red, green).
Let us hope Theros does not go down in Magic history as Return to Kamigawa. But even if it does, I will be playing it. There is a lot to love about the Kamigawa block, even if most of the cards are over-costed. Let us not forget that the Kamigawa block has some great staples (Gifts Ungiven, Sensei's Divining Top, Umezawa's Jitte) and some of the most flavorful cards (Marrow Gnawer).
September 19, 2013 update: I got to see videos of the white and blue boxes right before the poster made them private and it looks like the seeded boosters are only slightly different from a regular booster in that they contain cards representative of the set-meaning of any color..... I think that changes everything: all of a sudden I feel like entering the Thoughtseize lottery by going black. (Just kidding, blue is still the best color, even if the seeded booster pack is not nearly as seeded as the ones used for RTR and GTC. ) For the second color, I would rank red next, followed by green. Therefore, I plan to play U/R (Izzet) or U/G (Simic).
