White:
Leonin Iconoclast, while a little slow, fits right in with W/x heroic by giving them some much-needed removal. I'm ok with spending 4 mana to get 3 power in limited.
Mortal Obstinacy looks very playable as a heroic trigger that doubles as removal as well.
Blue:
Hubris is a great tempo play. If I'm heroic and playing a blue player and see that they have U/x up, I'm twice as terrified, because between this and
Voyage's End any combat trick/heroic creature/etc. becomes a risk.
Triton Shorestalker is good, but probably not
as good as people claim, unless you're drafting a tempo/heroic deck. That one toughness is a pain. It's in a better color than
tormented soul was during M13, so I'm more scared of it than I was the soul.
War-wing Siren is dope.
Black:
Bloodcrazed Hoplite will be dope if you can get him going on T3. Or he'll be a terrible topdeck.
Feast of Dreams is where it's at.
So is
Pharika's Chosen. Having access to
Sedge Scorpion too really gives the BG deck the space it needs to shine.
Red:
Bladetusk Boar is a little slow, but still a nice reprint. If you can get some of the cheaper removal to stop early Heroics, this is a good creature to ride to an easy win.
Magma Spray is a great addition to
Spark Jolt,
Magma Jet,
Rage of Purphorous, and
Lightning Strike.
Satyr Hoplite is another sweet addition to the R/W heroic deck. It now has relevant common heroic drops turns 1, 2, and 3, so I expect a lot more 16-land decks running 11+ heroic triggers in the format.
Green:
Ravenous Leucrocota is an interesting twist on
Giant Spider and other green 2/4s. Vigiliance + big butt is a strong combination in this block, as being aggressive while stopping aggressors is incredibly clutch.
Font of Fertility and
Golden Hind help green decks be green decks.
Overall:
White remains the strongest color in the set, while blue has enough beaters and tempo plays to be the best support color/the 2nd-best color overall. Red got a lot of things to stop or help white-based heroic decks, so I expect RW heroic to have an even greater presence in the meta. Black green decks have a chance to shine, and in general green is going to be bad unless you have a color that has an aggressive early game, or you're able to start ramping to your fattiess ASAP. Unfortunately, I don't see much of a change in the meta for green even if you ramp--it might have the most powerful creatures to play in later turns, but even more powerful voltron creatures are already on the board by then, so who cares? Black seems split between the aggro Minotaur deck and the grindier Gorgon/graveyard-based deck, which makes me feel like cracking packs hoping for one or the other is going to usually end with mixed results. I think there's now a chance for midrange decks to shine, but expect the format to be hyper aggro in the beginning, eventually turning into more midrange-style decks.
/ramble