Having played a few games of the RPGA's Living Arcanis campaign, here's my two cents:
- It's a great setting, but I don't think I'd consider it to be a "traditional" D&D setting.
- It's possible to create a PC using only the PHB, and drop them in (esp. if it's a non-spellcaster). But, there are an absolute ton of options in the Player's Guide to Arcanis, including new races, new classes, new feats, etc.
- Spellcasters can be considerably different from "stock" D&D.
--- Clerics have to choose a god to worship; each god grants his or her clerics special abilities. (And, the gods do not have alignments, so you could, in theory, have a LG and a CE cleric of the same god.)
--- Paladins can be of any good alignment. In addition to paladins, each god has its own "Holy Champion" class, with particular abilities relevant to that god.
--- There are race and gender restrictions to druids.
--- Arcane spellcasters have their own problems. The country of Ymandragore considers all arcanely-gifted people to be their property, and Ymandragore sends out hunters to track down arcanists. Thus, wizards, sorcerers, and bards have to be very careful about using their gifts.
- Races are considerably different from "stock" D&D, as well
--- Humans are pretty much the same
--- There's a human-like race called Vals (not dissimilar from aasimar); there are "bloodlines" of Vals related to each of the Gods, and each bloodline grants a particular set of powers. Vals were created by the Gods to watch over and lead the humans.
--- Dwarves are a cursed race; they used to be the Celestial Giants, and had the role that Vals have now. The Gods felt that the Dwarves failed in their task, and shrunk them to be smaller than humans. Dwarves are also banned from the afterlife, so there's some interesting rules on dwarves and death.
--- Gnomes are even more cursed; a gnome is a dwarf / human hybrid, and is usually an outcast, in addition to being physically stunted.
--- Dark-kin are a little like tieflings.
--- Elorii (elves) predated the humans, and the human gods wiped out 4/5 of the Elorii pantheon. As such, they tend to be holding a very deep grudge.

--- As talien mentions, there's a lizardfolk race, as well as half-orcs and half-hobgoblins.
- There is a ton of detail on the political and religious situation in Arcanis, and that can (and should) figure very strongly in an Arcanis campaign. The country a PC comes from, his religious leanings, etc., can play a very big part in that PC's development.