---The attacker can----:

Reverse transactions that he sends while he’s in control

Prevent some or all transactions from gaining any confirmations

Prevent some or all other generators from getting any generations

Double spend Bitcoins

---The attacker cannot---:

Reverse other people’s transactions

Prevent transactions from being sent at all (they’ll show as 0/unconfirmed)

Change the number of coins generated per block

Create coins out of thin air

Send coins that never belonged to him