Other : Playtesting
Given how modular and complicated the system is supposed to be one of my biggest fears is that it will end up having a HUGE number of problems out of the box. Sure, the BASE system will work but it will completely break at 4th level when Module 3 is combined with Module 5
A significant part of my concern is that they'll only release the core system (and maybe a couple of modules) for playtest, leaving the subsequent modules for later (and untested) books.
Ditto.For me, the biggest issue is Game Style. I can balance classes, change mechanics, etc, but if the game doesn't play in the style I want, I can't fix that, short of overhauling the entire ruleset.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.