What does the Juniper backdoor actually allow an attacker to do?

As the exploit gives root access to the Juniper device, an attacker can:

• Decrypt all VPN traffic to the device, thus allowing viewing of all traffic through encrypted tunnels that terminate at the Juniper box
• Edit firewall rules on the box, allowing an attacker to remove rules protecting assets within the network
• Alter routing tables where these are used, perhaps redirecting traffic
• View logs stored on the device
• Edit logs on the device to remove any trace of compromise

Root on the box.

So, "yes" to all.

http://kb.juniper.net/InfoCenter/index?page=content&id=JSA10713&actp=search

Exploitation of this vulnerability can lead to complete compromise of the affected system.

Update: Password disclosed
Rapid7 have disclosed the password.