Nothing. But many devices today do not support it anymore, instead they support S2Idle. S2Idle is more power-efficient than running-idle but less efficient than even Standby. Its huge advantage is waking up virtually immediately.
Linux has no problems running S2Idle but due to how S2Idle works, more components can wake up your system, therefore more components can misbehave, incl. devices attached by USB. Ugh.