If you're a moderately (or more) experienced role-playing gamer, Arcana Unearthed is for you. The following review is from an AU playtester who has been gaming long enough to remember the boxed D&D sets.
Let's look over the chapters:
Abilities:
If you've played D&D 3rd edition, not much is new to you here.
Races:
All new races that break from the Tolkien-esque mold D&D has adhered to from day one. Don't get me wrong, I love the Tolkien world, and the D&D races, but these are new, fresh, and flavorful.
Humans, faen, giants, litorians, mojh, runechildren, sibeccai and verik make up the land of the Diamond Throne. There is highly detailed information on all these races, including Savage-Species-like racial advancement for many of them. Combine that with the faen's ability to morph into a spryte, the transformation that is required to become a mojh, and the blessed runechilden, and you have races that aren't just selected during character creation and then stay the same. These races can evolve.
Classes:
Akashic, champion, greenbond, mage blade, magister, oathsworn, runethane, totem warrior, unfettered, warmain, and witch.
During playtesting I found the AU classes tend to have a fairly even power curve, whereas 3E classes can start somewhat weak, then get insanely powerful at high levels. It would take a long time to address each class, but I will say the champion, greenbond, and oathsworn are what the paladin, druid, and monk should have been all along.
The races and classes of AU have depth, deep roleplay potential, and kick butt mechanically. Overall, nicely balanced.
Skills & Feats:
You'll recognize most of the skills from 3x, with a few variations. The feats section is large and breaks down into talents, ceremonial feats, and general feats. Talents can only be taken at first level -- things like ambidexterity. Ceremonial feats tend to be fairly powerful and have a RP component; that is, the ritual involved to get the feat. There are many innovations here, and I especially like the new item creation feats. Instead of, say, 'scribe scroll', you have 'craft spell-completion item'. This adds flexibility and simplifies things simultaneously.
Equipment:
You'll see the old favorites along with many new items. There are exotic armors (indeed there are -many- new kinds of armor), and various weapon templates. One thing to mention here is that truly high ACs are possible in AU through normal armor, class abilities, magic, and spells. I feel that's an improvement over 3E where the warrior types could basically hit anything after a point.
Playing the Game:
Here is the bulk of the game mechanics. I'd call these rules like 3.25E, as they seem to be a hybrid of 3rd edition and the 3.5 revision. If you know 3x combat mechanics, etc., you can skim this section. If you're new to the system, it's all laid out here.
Magic and Spells:
Here is a huge innovation. Spellcasting has been standardized on one level, and made much deeper on other levels. Each type of caster gets spell slots per day, and a number of readied spells to choose from. This is kind of like how the 3x sorc handles spells, except your casters will -know- many spells, but only have a certain amount readied at a time. Don't like the spells you have readied? Sit down for an hour and swap them out. This system is flexible, and makes multiclass casters stronger than in D&D 3x, because slots stack. Nearly every spell has heightened and diminished effects. That is, if you cast the spell using a slot 1 level higher or lower, you get different effects -- very cool, and adds a fun new dimension to casting. Via feats (or rarely items), you can add spell templates to spells. There are many different kinds of templates like acid, lightning, corrupted, eldritch, holy, psionic, etc. Think of templates as a new way of handling metamagic feats. I should mention some of the old favorite metamagic feats are also still available in one form or another.
In conclusion, if you're a veteran gamer AU will provide a world of new possibilities whether you run a purely AU game, or add aspects of it into your existing 3x game.
Check out Monte Cook's website for free supplements, errata, and other useful AU resources.