The game itself has not changed radically, it is very much a recognizable AoW and it melds the best of both AoW1 and Shadow Magic.
As far as the races go, there are good in-game lore explanations for what has happened.
The campaigns focus on the conflict between the human Commonwealth and the Elven Court, so those races who generally inhabit only the inhospitable lands at the edge of the world (the polar regions, deserts etc) have been left out, but there is nothing preventing their addition.
This is most notable with Frostlings, who are immediately mentioned in the Commonwealth campaign. They are spoken of as being present in the world. Tigrans presumably similarly have been pushed out to the deserts and the edges of the more hospitable lands. No official word on that, though.
Archons were cut off from the world after the closing of the Shadow Gates when they joined the hunt for the shadow demons (who are, quite naturally, also gone, as are the syrons). Those who had been killed in the wars before and buried, well...
After Meandor's disappearance through the Shadow Gates (along with the other wizards), the aftermath of the fall of the Wizards and the disappearance of wizard magic in its old form threw the world into chaos and the wood and dark elves had to band together to survive. Julia and one of Meandor's most trusted lieutenants, Saridas, managed a reunification that healed the spiritual breach between the sundered kin and following that, the Elven Court of the High Elves was reconstituted.
Halflings have not been made much of, other than the writing contest that gave five people (myself included) beta access, so how their story is written in the future is still a mystery.
Nomads have been presumably absorbed into the human population, or they are again at the margins and thus not relevant to the central campaign.
Undead may or may not make a reappearance (probably not as a faction of dead animated by demonic spirits, as they were before), but if they do, it will be in a DLC. Necromancer has been mentioned as the strongest candidate for a new DLC class by Triumph.
You will not be going wrong with this game, but if you are hesitant, wait for reviews after release.
Member of the Evil Liberal Conspiracy - Moderate European Conservative