For the list of bugs fixed by Windows 2000 Service Pack 1 see the following Microsoft Knowledge Base articles:
There are already a set of "known issues" or gotcha!s for SP1. Kb article Q265381 contains a list of article numbers for release notes and known issues in Windows 2000 Service Pack 1. I would check back on this kb article for updates. I will pull out the real gothcha!s into separate tips but this is hit and miss since my sense of what is important may not match yours.
Windows NT service packs had to be reinstalled any time you twiddled with system - added a new device driver, added a network service, a network card, installed NT option pack. Windows 2000 theoretically has removed this requirement. You should never have to reinstall a SP under W2K. W2K is supposed to use cached and stored SP files to install or reset any files needed after install a new driver, nic card, or whatever. Significant enhancement and removal of a major gotcha! in Windows NT.