13th Age takes the OGL d20 foundations and turns it to it’s most photogenic angle and gave it proper lighting. All the things you expect from it are there, but they look and feel better.
13th Age is presented very well, and puts down into writing tricks that many GMs have implemented in their own games. The result of this is that new GMs, or GMs who feel very closely bound by the rules find a new kind of freedom in the story-focused mechanics that were absent from other d20 fantasy games. In some cases, I get the feeling that there are GMs that really feel the need for these things to be found in the Rules As Written in order for it to be valid as to destroy any doubt as to the source of their decision to improvise as opposed to “houseruling” the same ideas.
This doesn’t mean that 13th Age doesn’t bring anything new to the table. There are a few interesting mechanics, such as the Escalation Die, which could be an extra thing to look forward to in combat as a bonus to represent combat momentum is a neat idea.
Given this, it’s impossible to give 13th Age poor marks, but the challenge for it is how it can innovate beyond introducing story elements into d20. The metagame reasons for it’s design are sound, and the Icons are a great idea for helping GMs, but after applying lessons that have been in general circulation in non-d20 games for years I’m having a little bit of trouble finding how else it can come off as better.
That said if you like d20, and are looking to bring in more story elements into your game, then 13th Age is a no brainer. Get it study it and use what you can get out of it. Old hands at story games on the other hand might find 13th Age to be a little bit underwhelming as it feels like a “My First Storygame” manual for D20 fans.