Why is Ripple's effective genesis ledger at 32570? The documentation for Ripple says that the genesis ledger starts at 0 but the "effective genesis ledger" is at 32570, because some historical ledgers are "unavailable."

Why is this, and what does it mean for the average Ripple user?
It doesn't mean anything for the average Ripple user. In January of 2013, a bug in the Ripple server caused ledger headers to be lost. All data from all running Ripple servers was collected, but it was insufficient to construct the ledgers. The raw transactions still survive, mixed with other transactions and with no information about which transaction went in which ledger.

Without the ledger headers, there's no easy way to reconstruct the ledgers. You need to know the hash of ledger N-1 to build ledger N, which complicates things.