I heard that Michelson helps with avoiding re-entrancy bug. Why is that? Is there some feature in the language that prevent it?
To call another smart contract, you call TRANSFER_TOKENS.
The semantics of contract evaluation has been adapted (around alphanet, I'm not exactly sure when this happened) so that all transactions/operations such as TRANSFER_TOKENS are really executed after the contract evaluation returns.
This implies that the contract is not subject to immediate reentrancy.