Golgari: Dredge vs Scavenge
Azorius: Forecast vs Detain
Forecast had the potential to be a great mechanic, but the cards with the abilities aren't that great. Sky Hussar is probably the best one since its ETB trigger combos the best with the Forecast cost. Many other cards (like Govern the Guildless or Pride of the Clouds) just cost too much to be worth it. Yes, it keeps the card in your hand, but it still costs a ton. Detain, on the other hand, can easily screw with an opponent's field, even if it's just for one turn. The inability to block or attack is one thing, but the added ability to stop activated abilities is just icing on the cake. The fact that it also lasts until your next turn is those little sprinkles on top of that cake. However that cake is quite small since there are only 3 repeatable sources of Detain (Archon of the Triumvirate, Martial Law, and New Prahv Guildmage) and those sources are expensive. But, that's understandable given the power of the ability.
Rakdos: Hellbent vs Unleash
Card advantage? Where we're going, we don't need card advantage! Hellbent is a great mechanic since it works so well with red's lack of card draw and black's wanton discard that it flows so well. Hellbent is just awesome and there's not much else to say about it. Most of the cards with Hellbent are well worth it (the stuff from outside Ravnica kind of suck though). Unleash on the flip side isn't that great (starting to notice a pattern here?). It definitely works with the "suicide black/red" ideology, but it's still not that great. Not being able to block can and will hurt in the long run. The extra power and toughness the creature isn't really a good trade-off in my books. Some cards are worth it, but the majority aren't worth it because you have to live another turn to use the creature. If it gave haste, it'd be a ton better, but it doesn't.
Selesnya: Convoke vs Populate
Convoke works great in green because of its abundance of tokens. It basically gives token decks a second source of mana which is great. The best example of this usage is with Sprout Swarm being able to recycle the created tokens to cast the spell again. Populate also works great with green for the same reason that Convoke works. The only problem is that both mechanics rely on tokens (Convoke not so much), so a lack of tokens can leave you hurting.
Izzet: Replicate vs Overload
Replicate can be really funny. Like Storm, it has a natural anti-counterspell ability to it since you can make copies of the spell and those copies need to be countered (I have a feeling that Replicate was Wizards way to try and sneak a more balanced Storm back into existence). It also gives decent card advantage since it allows you play multiple copies of the spell but only having to use one card. Overload kind of does the same thing, but it doesn't have the same "natural anti-counterspell" ability. I think it's a little too powerful since the spell can't be turned against you with a Redirect or something similar. Overload gave blue a one-sided Shatterstorm (Vandalblast), a one-sided Swift Silence (Counterflux), and an almost better Plague Wind (Cyclonic Rift; yeah it doesn't kill anything, but it gets rid of everything).
Boros: Radiance vs Battalion
Radiance is one of those mechanics that's really a double-edged sword. While it can destroy an opponent's field, it can equally hurt your size as well. There are only 6 colours (colourless technically counts too, although it won't ripple through colourless creatures) in Magic, so there's a minimum 16.6% chance it'll hit some of your creatures too. Battalion, on the other hand, looks amazing. I know I'm a little ahead of myself here, but giving bonuses when you attack with several creatures is going to be fun. Lightning Helix every combat is going to be fun.
Battalion: 5/5 (preemptive)
Simic: Graft vs Evolve
Graft is a great mechanic since it snowballs into some really big creatures. It allows you to trade in your little early drops (like Simic Initiate) for bonuses on later creatures (like a Simic Sky Swallower). The problem is that the +1/+1 counters aren't really replenishable, so the effect is limited. Evolve looks like a similar effect to Graft, except in reverse. Graft can really be fast but Evolve looks like it'll be better in the late game. I personally think that it may be a little too powerful since cards like Wall of Ice will be able to power these cards up since its toughness will probably be bigger than most other creatures's toughness when it's played. It also affects all of your creatures, so it'll probably be very powerful.
Evole: 5/5 (preemptive)
Gruul: Bloodthrist vs Bloodrush
Bloodthrist (now a black mechanic) works well with green's trend to have creature superiority and red's ability to do direct damage. Buffing up a creature as a reward for a well-placed Lightning Bolt is great, an fortunately there are a lot of good creatures with this ability (especially since it made the shift to black). Bloodrush on the other hand, looks pretty bad. Mark Rosewater said it's basically going to be "Discard this creature and target creature gets +X/+Y until end of turn where X is the discarded creature's power and Y is the discarded creature's toughness" and I'm not a fan of it. If Rubblehulk is the representative of the effect and the creature's with that effect, it looks like they'll be a bunch of throwaway creatures that aren't going to be worth casting.
Bloodrush: 2/5 (preemptive)
Orzhov: Haunt vs Extort
Haunt sucks, plain and simple. It just sucks. Sure, you get to cast the spell twice, but it's really unreliable since you may not be able to control when the creature dies. Extort will suck in 1-on-1, but oh man, put that crap in multiplayer and you've got a winner! In a four-player game, an extra white or black mana on each spell for 3 life is definitely worth it. My main question is whether or not each permanent with Extort will stack (I doubt it, but it's nice to dream).
Extort: 5/5 (preemptive)
Dimir: Transmute vs Cipher
Transmute is a lot more powerful than people think. That Dimir Machinations in your hand? Yeah, it's actually a Vindicate. It always costs 3 too, which means you can start getting your perfect hand early on. However, you do have to have the right card to tutor for the right card. That Dimir House Guard won't get you any closer to that Vindicate you want. I'm not really sure what to expect of Cipher, but it seems like what Haunt was supposed to be. It will really depend on what the spell does, but I'm not sure that this card will be that great. Windfall on a stick doesn't seem like a Dimir ability and it may result in you holding back because you don't want to give your opponent card advantage. Honestly, it seems like just an "okay" mechanic to me. It has potential, to be sure, but without any other cards to back it up...
Cipher: 3/5 (preemptive)
Cipher: 3/5 (preemptive)