1. Devotion is awesome. I can't get enough of it, especially green devotion. Green-blue in Legacy with JTMS is a ton of fun, and in modern with Proliferate shenanigans it is also a ton of fun. Devotion has IMHO a lot of unexplored potential. Nykthos is by far my favorite dual land.
2. Strive is also awesome, and also has IMHO a lot of unexplored potential. NYX is a great set, not New Phyrexia great, but going that way. With a few new cards in the coming sets I believe Nyx can be a close second to New Phyrexia. Just like Kicker, Strive gets better in the late game, and sometimes insanely better.
3. Here and there cards that have a home in the eternal formats in the three sets.
4. The quality of some of the art, of course, is amazing.
1. Heroic is a card disadvantage mechanic. Poor Born of the Gods got hit with the Heroic stick.
2. Bestow, except for one or two creatures, is grossly over-costed; Saviors of Kamigawa over-costed.
3. The entire block is missing cards that would have made it a much more playable block. There is not a single card at common that even comes close to Delver of Secrets, for example. I like slow games, but not boring and dumbed down slow games, and a lot of games with Theros in them are just dumb. I love mono-black, but Theros made me not want to play mono-black on account of the one-trick pony Gray Merchant of Asphodel.
4. I love Greek Mythology, but the coverage of ancient Greece in Theros felt 'high school.' Even though the Kamigawa block takes a lot of heat for playability, I feel that Kamigawa is more Japanese than Theros is Greek. I would have liked Theros to feel more Greek.