But, considering there is only so much money that startups have access to, and most early employees are not primarily motivated by money (I would hope, or else they are likely quite naive), the biggest let-down I've experienced as an early employee, that would have made up for the lower comp, would be actual access to the founders' networks and learning opportunities.
If being an early employee meant getting to meet with investors and help the founder raise money (and make an impression on those investors), if it meant being actively involved in the struggles of the founders involved in running a startup, then I suspect those of us who wish to start our own startup some day would consider that a reasonably attractive opportunity to really learn those skills and build their network.