Botnets with their ability to aggregate and coordinate hundreds of thousands of compromised computers into a wide-array platform for DDoS assaults capable of shutting down providers, companies and even governments.
Of particular concern in the Arbor report is the finding that attack traffic is growing faster than ISP capacity -- attack volume may be more than twice the data capacity of even recently upgraded backbones.
The entire Infrastructure Security Report can be downloaded here. (Free registration required.) It's scary, important reading.