Another Exchange specific issue: we had a problem after migrating to Exchange 2010 and then uninstalling Exchange 2003, eventhough replicas of public folders were moved to the new server. Errors would show in the Exchange System Manager, and the following message would occur:
MapiExceptionADPropertyError: Unable to mount database. (hr=0x80004005, ec=2418)

There is a excellent post on TechNet Forums about this topic. However, it does not include information for Exchange 2010, and mentions to restart the Information Store, when restarting the System Attendant is sufficient. Corrected instructions below.

Open ADSI Edit, connect to a Domain Controller, change the context to Configuration.

Create the Folder Hierarchies under the Exchange Administrative Group

1. Navigate to Configuration ⇒ Services ⇒ Microsoft Exchange ⇒ <Exchange Organization> ⇒ Administrative Groups
2. Right click on <Exchange Administrative Group> and select New Object
3. Select msExchPublicFolderTreeContainer as class and click Next
4. Enter the following as value: Folder Hierarchies, click Next, Finish

Create the Public Folders Tree Object

1. Right click Folder Hierarchies and select New Object
2. Select msExchPFTree as class, click Next
3. Enter the following as value: Public Folders, click Next
4. Click on More Attributes button, select msExchPFTreeType and set the value to 1
5. Click OK, Finish

Populate the msExchOwningPFTreeBL attribute object of the PF Store

1. Double click the newly created “Public Folders” object
2. Double click distinguishedName, copy the value to the clipboard, click Cancel
3. Exchange 2007: open properties of Configuration ⇒ Services ⇒ Microsoft Exchange ⇒ <Exchange Organization> ⇒ Administrative Groups ⇒ <Exchange 2007 Administrative Group> ⇒ Servers ⇒ <Affected Exchange Server> ⇒ Information Store ⇒ <Storage Group> ⇒ <Public Folder Database>
4. Exchange 2010: open properties of Configuration ⇒ Services ⇒ Microsoft Exchange ⇒ <Exchange Organization> ⇒ Administrative Groups ⇒ <Exchange 2010 Administrative Group> ⇒ Databases ⇒ <Public Folder Database>
5. Double click the msExchOwningPFTree attribute, paste the value that was copied to the clipboard in step 2
6. Click OK twice

Try to mount the Store

1. Restart the Microsoft Exchange System Attendant Service
2. Open Exchange System Manager and try to mount the PF store

If you work in the IT industry, take a moment to think about all the hours you have spent following best practices, and what is commonly called hardening of a server system. One of the most common practices is to only run the bare minimum of services and thus reduce open ports facing the network.
Microsoft seems to have a slightly different take on this: in the official Exchange 2010 Prerequisities, admins are instructed to – amongst others – install a Windows Server Role named Web-Server. What they do not mention, though, is that this component also installs the FTP server and IIS Core components. Both of which are not needed for Exchange. If you happen to want only the minimum required installation, be sure to swap Web-Server with Web-WebServer, which omits the slack.