Legion Warboss is a worse version of Rabblemaster, Laelia is almost certainly the most powerful red 3 drop of all time. Sin Prodder often led to less than optimal outcomes, Combat Celebrant is more flexible; going in both mid range and zerg rush decks. Moving some slots out of the 3 drop slot into 4 to give mid - late game red decks more options.
Black had too many double cost 3 drops. Nighthawk Scavenger is a strict upgrade, Feed the Swarm for more powerful spot removal, Oona's Prowler didn't really go in aggro decks, and never really fit in as a viable discard outlet, Morbid Opportunist is a flex 3 drop. Tokens need more sac outlets after the success of Carrion Feeder and Flesh Carver fits the bill.
Green is getting a lot more graveyard interaction, as well as an additional (powerful) mana dork.

Extra draw engine on a stick +1

Token buff #1. Carrion feeder is a finisher, Sac outlet and minigame starter. Blood artist gives extra pressure to trading resources.

Countryside Crusher didn't get played, Irencrag Pyromancer almost certainly will. Spirit of the Labyrinth didn't get played, Serra Paragon has graveyard interaction. Graveyard interaction is shockingly powerful. Green turned out not to be the token color, and two mana for a 1/1, even repeat ably, is a bad rate. Replaced with an immensely powerful 4 drop.
Tolarian Academy is being taken out because: It's really a blue card, it only goes in one deck, at its best it's really too good for my cube, and at its worst its useless. Gemstone Mine is more 5 color mana fixing, and an indirect buff to Crucible of Worlds.