I'm to understand that little is known of the man but I'm curious as to exactly what kind of man he could have been given some of the feats he accomplished. I know that much of his success is owed to Rome's lack of response to his rebellion, sending second-rate troops to fight him. But still, he takes an army of people from all sorts of backgrounds and cultures and consistently trounces the Roman army, including one battle fought on two fronts in which he defeats both forces.
How does a gladiator, a showman, have the military skill and leadership to win such victories against Rome itself?
We know very little from ancient sources--the Romans don't seem to have been particularly eager to delve into the Servile Wars--but Appian and Florus both identify Spartacus as a Thracian soldier who served as a Roman auxiliary before his eventual desertion/mutiny landed him in gladiatorial school. So that might account for a least some of his skill at warcraft, especially if he advanced to some rank of centurion (that last point is pure speculation, btw).
The classicist and former tls editor Peter Stodhardt recently published a book called The Spartacus Road. While it is as much a travelogue and memoir as it is a historical account of the rebellion and Roman slavery in general, its essayistic style is well-suited to consider some of the unanswerable question you're asking. For a more straightforward account, you might try Barry Strauss's The Spartacus War. Be forewarned though--he's an arch conservative (I never read the book, but his leanings definitely bled into a few reviews of his I edited some years back).
Edit: can --> might