First we have the Conclave of Selesnya, the green/white guild of Ravnica. They value the Oversoul, nature, strength in numbers, so naturally if you play a token generator, this is the guild for you.To aid your token swarm, there is the new mechanic, Populate, which allows you to produce a copy of a token you already have. With a land becoming an 8/8 token, and token generators from previous sets, (Increasing Devotion I'm looking at you) your token generation gets even faster and more powerful. Like with green/white, there is life gain installed somewhat in it, but in my opinion, there needs to be more. True, the legendary creature, Trostani, Selesnya's Voice, gives you life equal to creature's power when it enters the battle field, other than that and a few other cards, life gain is few and far between. Over all though, I'll admit to liking Selesnya on a whole due to the fact it's everything it either token generation of life gain. Now, I'm waiting for them to reprint Soul Warden or Essence Warden.
On the contrast to nature loving Selesnya, next we have the mean, party loving red/black Cult of Rakdos. In this set, they have creature destruction, and even a card that allows you destroy planeswalkers(Dreadbore) along with burn spells to help summon the guild leader, Rakdos, Lord of Riots. He is a 6/6 flying trample for two black and two red, but someone has to take damage before casting him, but with the Rakdos mechanic of Unleash, which allows players to put a +1/+1 counter on the creature in exchange for it not being able to block, damage comes easy to those creatures. Honestly, the unleash mechanic is weak just due to the fact that next turn the opponent could have a creature as strong as the one you unleashed, and the fact that they don't have haste. If they made it a bit stronger, then Rakdos could potentially dominate standard more.
Next we have the scientists of Ravnica, the Izzet, led by Niv Mizzet, the smartest being on the plane of Ravnica. While blue/red might seem like a contrast at first, once you delve into the Izzet guild, you'll find that these colors provide for a wonderful counter spell deck with a hint of burn. The Izzet mechanic of Overload makes it so that a spell that only affects one creature, affects each creature that either you or opponent control, depending on the effect of the card. Niv Mizzet, Dracogenious provides for pinging your opponents for a damage while yo draw a card. I'll admit that that he is not as powerful as his counterpart, Niv Mizzet, Firemind, but still provides for interesting gameplay.
The Azorius serve as Ravnica's justice system, and what better to control the people than blue/white? Azorius uses the mechanic of Detain, which prevents target creature from activating activated abilities or from attacking or blocking. While the mechanic might sound amazing at first, you then realize that it's only for a turn that they're detained only for a turn, and next turn they're up and going again. I honestly don't like the mechanics because it just slows the opponent down, but if you can't keep up the detainment, then you will die. Isperia, the legendary creautre, just detains creatures whenever she attacks, but other than that, compared to the other legendary creatures, she is severely underpowered. Azorius is good to slow the game down, but if you're looking to play aggro, then Azorius is not for you.
Lastly, we have Golgari, the green/black guild that believes that in in death, there is life. They run on a mechanic called scavenge, that allows the player to pay the scavenge cost when the creature is in graveyard to put +1/+1 counters on a creature on the battlefield equal to the power of the creature being scavenged. The creture being scavenged is then exiled. Honestly, I'll admit, I hate Golgari with a passion. True they work on graveyard grab so that you can't truly kill them, but I just hate the way it runs. Maybe I just don't value my stuff in the graveyard and prefer to keep my creatures alive as long as possible.
Overall, I am impressed with the Return to Ravnica block so far. It provide a different balance than Avacyn did, but it can still be used in different decks. Personally, I think standard just got a bit more fun, and this provides good chances for people to make multi-colred decks. Now, if only Gatecrash would hurry up and get here.