An odd network bug
I did encounter one odd network bug in SP1, but as far as I know, it affects only my setup; I haven't heard about it anywhere else. On my home network, which uses a Linksys WRT54GX4 router, I can't connect to other Vista PCs from my SP1 system when I use wireless networking. I can, however, connect wirelessly to other XP PCs. And if I connect via Ethernet, I can connect to both Vista and XP PCs.
Equally odd is that I can ping other Vista machines from the SP1 machine without any problems. And I can also take remote control of another Vista PC, using Remote Desktop Connection. However, I can only take remote control of it if I establish a Remote Desktop Connection using the machine's IP address rather than its name.
Other changes
You'll find occasional cosmetic changes here and there in SP1. For example, BitLocker encryption lets you choose which drives to encrypt. In addition, the Disk Defragmenter (Control Panel --> System & Maintenance --> Defragment your hard drive) now lets you choose which volumes to defragment.
Aside from these, though, most changes are under the hood. Microsoft touts a variety of performance, security and stability improvements, as well as other plumbing changes. For example, SP1 supports the ExFAT format, a new format for flash-based devices that supports larger files and more files per directory than is currently possible. Microsoft also claims that SP1 is more stable than non-SP1 Vista and reduces system crashes. It also says that SP1 is more secure.
For a more complete list, see Microsoft's document Overview of Windows Vista SP1.
The bottom line
Sometime in March, you'll be faced with the decision of whether to upgrade to SP1 when it becomes available to the general public. Should you do it?
I'd say yes. The elimination of the Kill Switch alone is reason, by itself, to upgrade. If it's true that Vista will be more reliable and more secure under SP1, that's another good reason as well. If Microsoft is to be believed, most people will get performance benefits as well, although on my system, I found copying to be slower, rather than faster. Still, I'm keeping SP1 on my PC. The slowdown in file copying is a minor annoyance, and it's a small price to pay for ensuring that the Kill Switch will never go into effect.