I want to design the RPG that I want. The OGL exists, in its original d20 and its Paizo forms, and plenty of Open Gaming Content exists that can be incorporated.

Things I want my game to do:

1. Quick start, fast learning curve. Character generation and running your character's mechanics should be things new players master in one session.

2. Versimilitude. Players (and the GM) should believe that the world would continue in some form when the PCs are offscreen.

E6 lends itself to #2--the PCs become powerful in the world, but never powerful enough to overwhelm the world.
E6 lends itself to another design principle--with only 6 levels, I can benchmark things (spells, abilities) in three tiers (common, uncommon and rare). That helps 1st, 2nd and 3rd level spells to feel different, and to have design principles that make them 1st, 2nd, 3rd level besides just inflating numbers.