Probably the ease to which it spreads, it's elusiveness, it's ability to update itself.
It's pretty sophisticated, from what I've read.
That's correct. Specifically.....
Multiple attack vectors -- it can spread through USB sticks, Windows vulnerability (MS08-067), and Windows shares (bruteforcing passwords)
Communication and updating -- it uses a peer-to-peer protocol to communicate and update itself
Cleans up -- it patches the vulnerability it used to gain access (but it patches it differently from how Microsoft does it -- that's how we can
detect it remotely)
Difficult to remove -- it disables antivirus and blocks access to Windows Update, Antivirus vendors, security sites, etc (can also be used to
detect it locally)
Mysterious -- because of the automated updates, nobody knows what the functionality is going to be.
<edit> it's also what I would consider the biggest worm since the early 00's (2003/2004), and it's much less obvious than others (Blaster/Sasser used to be obvious, because it crashed the service -- Conficker doesn't)