Even though Active Directory was scalable enough to meet the needs of most organizations, there were some improvements to be made after several years of real-world deployment experience. Many of the functionality differences with Windows 2000 are the direct result of feedback from AD administrators.
As with the new features, we suggest you carefully review each of the differences and rate them according to the following categories:
It would positively affect my environment to a large degree.
It would positively affect my environment to a small degree.
It would negatively affect my environment.
The vast majority of differences are actually improvements that translate into something positive for you, but in some situations, such as with the security-related changes, the impact may cause you additional work initially.
Unique security descriptors are stored once no matter how many times they are used as opposed to being stored separately for each instance. This alone can save upwards of 20%-40% of the space in your DIT after upgrading. Note that an offline defragmentation will have to be performed to reclaim the disk space.
Several bugs have been fixed which erroneously caused user lockouts in Windows 2000. A new Active Directory Users and Computers property page called Additional Account Info and the lockoutstatus.exe utility are great troubleshooting tools for diagnosing lockout problems.
There are several new event log messages that will aid in troubleshooting replication, DNS, FRS, etc.
Replication in Active Directory is done at the attribute level. That is, when an attribute is modified, the whole attribute is replicated. This was problematic for some attributes, such as the member attribute on group objects, which could only store roughly 5,000 members. LVR replication means that certain attributes, such as member, will only replicate the changes within the attribute and not the contents of the whole attribute whenever it is updated.
The previous default was 5 minutes, which has now been changed to 15 seconds.
With Windows Server 2003, whenever an attribute is added to the Partial Attribute Set (PAS), a global catalog sync is no longer performed as it was with Windows 2000. This was especially painful to administrators of large, globally dispersed Windows 2000 domains.
Instead of sending LDAP traffic, including usernames and passwords, over the wire in plain text with tools such as ADUC and ADSI Edit, the traffic is signed and therefore encrypted.
The algorithms used to generate the intersite connections have been greatly improved to the point where the previous limit of 300 to 400 sites has been raised to support roughly 3,000-5,000 sites.
With Windows 2000, whenever you disabled the global catalog on a DC, the global catalog removal process could only remove 500 objects every 15 minutes. This has been changed so that the process is much quicker.
The DLT service can be the source of thousands if not millions of linkTrackOMTEntry objects that are nestled within the System container of a domain. By default, the DLT service is disabled on Windows Server 2003 domain controllers.
To enhance security, the Everyone security principal no longer means all unauthenticated and authenticated users. It instead represents only authenticated users. To grant the equivalent of anonymous access in Windows Server 2003, the Anonymous Logon account should be added to the Pre-Windows 2000 Compatible Access group.
If you find that more than two or three of these would benefit your environment significantly, and fewer than one or two would have a negative affect, that is another good indication that an upgrade to Windows Server 2003 would benefit you enough to start in the near-term. This is by no means a hard-and-fast rule, since some features or differences may be more important than others. For example, if you have over 300 or 400 sites with domain controllers, the improvements in the KCC could potentially help you out significantly. Likewise, if you see the need to add attributes to the partial attribute set in the future, and you have large geographically disperse global catalog servers, then the no global catalog sync behavior could save you some long weekends babysitting replication. You may view other features, such as the MMC enhancements, as benefit, but not to the same degree as the other two just described. You'll have to weigh the priorities of each when you are considering them.