I recently pre-ordered the 13th Age RPG, and I've had a chance to look through the rules and come away with some thoughts.
In the same way that Pathfinder is D&D 3.75, you could say that 13th Age is D&D 4.75. It builds on many of the ideas of 4E, and tries to improve them. At the same time, it simplifies a lot of the mechanics, moving the complexity away from the optimal choice of feat and items to the storytelling. This could be either fun or frustrating, depending on the GM and players.
A lot of the storytelling is built right into the character mechanics. For example, each player receives three or more relationship dice. These are assigned to positive, negative, or conflicted relationships with the thirteen icons. The icons represent the most powerful NPCs in the game world, and can be heroic, ambiguous, or villainous. The relationship dice can be used a number of ways. They're rolled at the beginning of a session to determine which icons (or more often, which icon's organization) will be influencing the session, or they may be rolled as a result of an encounter with an icon's organization.
In addition to icon relationships, storytelling may have mechanical effects. Some class abilities depend on the player telling a good story, and leaves it to the GM to judge whether they get a bonus. It's also up to the player to convince the GM that his background applies when the situation calls for it.
Anyone who's played 4E will notice similarities, and some differences. Each player gets a certain number of recoveries (healing surges, though you're supposed to roll for the amount of healing), an AC, physical and mental defenses, HP equal to three times a base plus their con modifier, and a feat. There are bonuses to stats based on both race and class (each of which can apply to one of two stats).
Instead of skills characters get backgrounds. A background is more a description of a profession than a skill: something like court jester, temple initiate, or cat burglar. Each character starts with 8 points to spend on backgrounds, with a maximum of 5 in any one background. You add those points to any skill checks which are relevant to a background. The DM usually picks the attribute to use (for instance, is it a dexterity or intelligence check?), and the player makes the case for which background is relevant.
Each of the classes also gets 3 talents, and most of them get powers.
Talents generally customize your characters, by giving a permanent bonus or allowing you to do new things (substitute Wis for Cha in all your class abilities, for instance). But they can also give you what are effectively powers.
Powers are a lot like 4E powers. They are usually at-will, recharge, or daily. Recharge powers are similar to encounter powers in 4E, but when you take a quick rest, you have to roll to see if you recover them or not. You are usually free to choose as many powers of each type as you want, up to your limit, so it's not like 4E where you have a certain number of at will, encounter, and daily powers at each level. The classes are different in the type of powers they receive. All fighter powers (maneuvers) are at will, and they take the form of flexible attacks, where you roll your die first, and then decide which maneuver to use based on what conditions the natural roll gives (some maneuvers trigger on odd rolls, some on even, some on hits, some on misses, some at 11+ or 16+). Rogue powers are all at-will, but sometimes require momentum (that you've hit an enemy and haven't been hit by one since).
Barbarians, rangers, and paladins are considered simpler classes to play, since they don't have any powers, just talents (though the number of talents they get increase by level, unlike other classes). While clerics, sorcerers, bards, and wizards are more complex to play, since their powers are spells, and are often recharge or dailies (bards also get songs, which are sustainable spells with finishing effects, and battle cries, which are flexible attacks).
Feats are usually tied to talents or powers, increasing their effects. This can be challenging when spells, which can be changed each day, have associated feats. The rules encourage the GM to be flexible in letting players reassign feats, but leave it up to him to decide. All spells can be increased in level, filling a higher spell slot for more powerful effect. Generally, you only get spell slots at two or three levels, losing lower level spell slots, so you need to trade for the higher level versions of spells if you want to keep them.
There are only ten levels, but there's a significant advance with each one, each level giving you a feat, +1 to defenses, attack, and ability/background rolls, and another weapon die (so your basic attack does level x d6-d10). Levels are divided into tiers, with adventurer (1-4), champion (5-7), and epic (8-10).
In addition, after each session you get an incremental advancement, receiving part of your gains for the next level now.
Combat in 13th Age is considerably more abstract than in 4E. Enemies are either nearby (in which case you can reach them in a move action) or far away (you have to move to become nearby first). However, it is possible for some creatures to be behind others, in which case, the ones in front, as long as they aren't already engaged in melee, can intercept you if you try to attack the ones behind. The biggest constraint on movement is that you need to engage someone in order to hit them with a melee attack. You can't move after that without first disengaging (which requires a roll) or taking an opportunity attack. Since you have to move in order to engage, you can't attack someone else without disengaging. Of course, more than one enemy can engage you, so you can still be engaged with multiple foes at a time. Some classes have ways to disengage. For example, the fighter can intercept even when he's engaged, disengaging from his current enemies, and the rogue has numerous abilities that let him disengage.
But probably the most significant addition to combat is the escalation die. Every round after the first, the DM increments the escalation die, starting at 1 and increasing each round until it reaches the maximum of 6. Certain powers key off the escalation die (wizards' cyclic spells aren't expended when the die's even, for example). But the main effect is that the PCs add the escalation die to their attack rolls. Only the most dangerous monsters do the same. This gives the PCs a significant advantage as the fight wears on, but they have to earn it. The DM is cautioned not to advance the die if the PCs are hanging back rather than engaging the enemy.
I haven't had a chance to play the game yet, just to read through the rules. It looks like there are a lot of interesting ideas here, some of which sound really fun, and some of which I have my doubts about. But at this point, I'm eager to try playing it, and that's as high a praise as I can give so far.