PPTQ's Proximity to other large events.
Wandering in late, to provide the ‘O’fficial answer, which is close to most of the unofficial ones… :p
The link Adam posted is clearly official. Prerelease weekends are the only absolute “blackout” dates, but it's recommended that you avoid scheduling against nearby GPs, RPTQ, or PPTQ.
On that note - a lot of the stores in the Front Range area (Denver plus an hour or so) are cooperating as much as possible, via a Facebook group. I think it's really helped; there's an event just about every Saturday and Sunday but not a lot of tough choices on which one to play. It's hard to get store owners to do this, but you can tell your locals about our success, maybe it'll work there, too.
d:^D