I was led to the OSR through a winding course that started with Castles and Crusades, then Basic Fantasy, next Swords and Wizardry and finally to Labyrinth Lord and Delving Deeper.
However, 13th Age over the last 18 months has become what I most want in a RPG with DnD as it's pedigree.
That being said, many of the games I run include new players ranging in age and while there is nothing too complex about 13th Age, sometimes I just want to speed them through character generation and not overwhelm them with choices.
That's why I began working on 1st Age, to take the core of the Archmage Engine that powers 13th Age and apply it to a leaner set of rules, like Labyrinth Lord. No feats, no talents, less fiddly bits overall. Bits that aren't bad or poorly designed, but that sometimes are just in the way.
So if 13th Age is a "love letter to DnD", 1st Age is my fan mail to Gygax, Arneson, Moldvay, Cook, Tweet, Heinsoo, and Proctor.
A leaner version of the Archmage Engine, for those times when you want (or need) to be faster and looser.