The Age of Shadow is mostly identical to stock Openquest (including large chunks of reprinted text), so I'm mainly going to be discussing the differences. The differences include: a sample of play, character creation, barter value, some minor clean-ups and clarifications in the combat chapter, some changes to the spellcasting systems available, slightly different character improvement rules, and the bestiary.
In reverse order:
The bestiary's formatting is superior to stock Openquest's. It compresses more information more usefully than stock Openquest monster profiles, and its formatting should be adopted or improved upon in Openquest 2nd edition. It contains 15 monster types, with three stat profiles for each (lesser, common and greater) plus an animal list that looks like it came from stock Openquest. The monster types are mostly staples - beastmen, vampires, barbarians, sorcerers (it's nice to see some human antagonists written up), demons, etc. It also has a list of animals statted up, though this appears shorter than the list in stock Openquest.
I like the horde rules, which simply codify what happens already when you run a large number of antagonists. I don't like the Fear rules. 10 of the 15 monster types in the bestiary have the Fearsome special rules apply to them. You make a Persistence test when you first encounter the creature (and RAW appear to test for each creature, not even each type), and then depending upon whether you passed or failed, critically succeeded or fumbled, you cross reference your result with the Fearsome rating of the creature to determine the result, which includes penalties, fleeing, passing out, etc.
I would have preferred only one kind of Fearsome, and to have its effects be to make a Persistence test when a group of creatures with at least one Fearsome member is encountered. Fumble and you pass out, fail and you flee until you can't see any Fearsome monsters, pass and you stand your ground, and critical and you force the Fearsome creature to make its own Persistence test against you as if you were Fearsome (A creature that expects you to flee before it suddenly freezes up in shock when you seem not only unaffected but actually ready to take it on).
The character improvement rules grant 2 x 1d4% to skills, or 2d4% to one skill, instead of the flat +5% to one skill that Openquest does. The average gain is the same, but there's more variance. I don't know how I feel about this idea, since the problem I've had with the 5% gain from spending an improvement point is the rapidity of improvement, which I don't see this affecting. I'd be curious about the reasoning behind it.
And no, you still can't improve SIZ.
The spellcasting systems basically remove divine magic and make it harder to cast sorcery and innate magic aka battle magic from stock Openquest. You have to spend background points during character creation to be able to cast spells. Sorcery is basically stock sorcery with a corruption mechanic - fumble and you gain a point of corruption, get more corruption than your POW, and you become a evil NPC. You also tend to know fewer spells than a sorcerer will in stock Openquest.
The goal of these rules is to encourage low levels of magic, as part of creating a low fantasy feel. I'm not totally convinced this is possible in any game with PC spellcasting and defined spells. This is a larger discussion than this review can accommodate, and it's not a problem unique to Age of Shadow by any means, but I think that what people mostly want when they say they want "low fantasy" or "rare magic" is for magical spell effects to feel less like a form of technology. This means: less reliable, less safe, less defined, and to create a sense of awe and wonder at both its operation and its means. IRL, the essence of "magical thinking" in the developed Western world is the elision of means by which something is accomplished. Any game that defines and explains magic's operations will have magic that is more technological than "magical".
Anyhow, corruption triggers extremely rarely, and sorcerers are characters who will have high POW scores or who will spend improvement points to get high POW scores as rapidly as they can. I'm not sure how much of a threat corruption would actually be, except over extremely long play. As well, the only mechanical effect is to lose the character, which is a type of effect I hate (especially since I have played, and will play again in future, evil characters). I don't think this represents corruption very well - there's no gradations, just a threshold that if you cross, you lose the character.
The combat chapter has some minor rewrites, particularly around moving (He basically shows the action-move-reaction schematic a little more clearly). There are no differences in the rules, but I think this chapter reads easier than stock Openquest does, and I commend Kristian Richards for caring enough to clarify some of these minor issues.
Age of Shadows does not use money. Instead, all objects are assigned a barter value. BV is basically a virtual monetary system. I think this works as a transitional step to representing a non-monetary economy, but it's a virtual monetary economy, not truly a moneyless one. The end result is that you'll carry around physical treasure with a certain BV rather than a small cargo container worth of coins. For some people, this is far enough away from money to get across the Tolkienesque feel. Personally, I'd prefer something more radical.
Also, the gear list is the same boring OGL list as every other fantasy game these days. I've complained about this before.
Character creation is similar to stock Openquest except that it now includes elves, dwarves, and some "background points" (Humans get 3, Dwarves 2, Elves 1) that allow them to boost skills or cast magic, etc. Elves and Dwarves get some fiddly modifiers as powers, and can cast spells. Background points are spent like improvement points, or to allow humans to know and cast spells. I wanted to like and praise this stuff, but I mostly find it bloodless and confusing. As many of my readers recall, I abolished all attribute differences and most special powers that different species / sub-species had. While perhaps not suitable for a Tolkienesque world like Age of Shadows, I would suggest that attribute variances are the wrong way to handle this, as are powers that are merely +25% to this situation. I've come to prefer treating species abilities are unique features that don't add mechanical benefits directly, but that allow them to do something or use a skill or other ability in a way that no one else can.
Overall, I think it's worth downloading Age of Shadows, but I would hold off on buying it. The main selling point is the new formatting for the monster stats, which I encourage the rest of the Openquest community to adopt and use.