So, please take it as a given that 13th Age is going to be a great game. It's got some great minds behind it, and it really feels like it takes d20, combines a few of the good lessons from 4e, and makes a "good parts version" of d20. The last d20 game that made me stand up and take notice to this extent was Blue Rose (the precursor to the excellent True20 line from Green Ronin).
So, given that, it's still curious that the most powerful idea in it has very little to do with the rules, and that is Icons. You can read more about them at Pelgrane's site (and in other place - it's a popular topic) and I want to draw a circle around it as an important idea that's going to see a lot of emulation down the line.
The Icon model is a logical extension of the idea of NPCs as setting. This is not a new idea, but it's a very clever implementation of it which presents the idea so clearly that I suspect it will become the common parlance for the concept. In short, there are 13 powerful, iconic being in the setting and each PC is connected to at least two of them (for good or ill). These icons are tightly tied to the setting - so much so that the setting itself can be sketched rather thinly around them. They are not remote beings or gods - they are tightly tied to the day to day world, and the tie to the PCs means that PCs are similarly close to the centers of power.
At first glance, this is interesting, but maybe not compelling. However, there are some subtleties baked into this that really flesh it out.
The first, and probably most subtle, is the fact that the connection does not always manifest directly. This is backed by the mechanics (you can call upon a connection in situations where the icon would never just show up) but the concept is straightforward - that connection implicitly includes a connection to the entirety of that Icon's "faction" - whatever organization, allies or otherwise they may have. And note, those factions are loosely sketched at best - they're an avenue for GM and player creativity, which is a nice bonus.
Now, in the hands of a lame GM, this could be an excuse to undercut the whole connection mechanic, by perpetually keeping PCs at arms length from the Icon in the worst traditions of clan-based play, but the risk of misuse is the price you pay for any good tool. As presented, it is a means to flesh out the setting in line with player needs AND to draw the player into the world.
The second thing is that it drives a very interesting choice: the game does not guarantee that PCs will be powerful, but it does guarantee that they will be prominent. Not to say they can't also be powerful, but by necessity, they will be drawn into matters of grave import, as absolutely suits the particular flavor of fiction that 13th Age embraces. This is an upshot of the icon-centric setting design, and it's pretty powerful mojo.
Now, I mean no sleight to the specific Icons of the 13th Age setting, but I know that my very first instinct is to build my own setting around a different set of Icons, and I suspect that impulse is far from uncommon. In addition to building an interesting, playable world, 13th Age is presenting a tool for setting design which - to my mind - pushes setting technology forward dramatically. Other games (Dresden Files, Burning Empires) have made similar pushes, but 13th Age has managed to do it in a way that is easy to illustrate, explain and (most importantly), re-use. That is a big deal, and I am duly impressed.
Which is, of course, no reason not to hack it some more. But that's another post.
1 - D20 evolution has an interesting cycle which I will grossly generalize as follows: A small number of games push the boundaries of what the game is, and a larger number of them expand and refine on the model. Games like Blue Rose and 13th Age push things, and things like Pathfinder refine them. This does not make the "push" games better - refinement and expansion is also essential - but it does make for a difference in what to expect from the game. It also invites debate regarding which games push and which ones refine, and there's a good chance that the ones that a given player think push are the ones the like best, but that's neither here nor there. The bottom line is that I feel that 13th Age pushes d20 forward, and (assuming they feed back into the OGL) improves the technology for everyone.