With large organizations, the most likely problem is that the import process runs into a timeout when it updates the user mapping. Update Rollup 8 contains a fix for this problem which has to be enabled manually. See http://xrm.ascentium.com/blog/crm/Post768.aspx.

Another common issue is that the import fails with the following error

ExecuteNonQuery requires an open and available Connection. The connection’s current state is closed.

This error is likely caused by an issue in the .NET 2 Framework. See this article of the EMEA CRM Support team for a description
http://blogs.msdn.com/emeadcrmsupport/archive/2009/10/12/error-while-importing-an-organization-executenonquery-requires-an-open-and-available-connection.aspx

Another cause of this issue are reports which could not be published to the report server by any reason. For example, reports which are spiced up with custom assemblies cannot be published without the referenced assemblies. The import will fail when it tries to publish such a report. In this case enable the tracing, which generates a trace file for the mmc-console. With help of this file you can identify the report which is making problems. To solve the issue, you can either delete it directly from the database (unsupported) or you delete it prior to the backup of the originating system. After the successful import, you can import the problematic report again.

See this thread in the CRM Deployment forum for further description http://social.microsoft.com/Forums/en/crmdeployment/thread/1336bb5d-a119-44e3-83dd-1f557edccc31

The language code 1033 is not a valid language for this organization

I searched the net, but the only things I found was that this error is listed as an error code of Dynamics CRM (see http://msdn.microsoft.com/en-us/library/bb930493(loband).aspx) and a thread in the Dynamics Forums (http://social.microsoft.com/Forums/en/crm/thread/70bbbf36-1ecb-443e-852a-bebaa70c7206), but sadly without a solution.

After contacting the Dynamics CRM support team, the cause of this error was found in a matter of minutes. The system was redeployed from an installation with German base language to a system with English base language. This is an unsupported scenario.

Neither there was a warning at the redeployment wizard, nor is this mentioned in the Implementation Guide (at least, I haven’t found it there). The support engineer provided me following excerpt there this scenario is described as unsupported

The original deployment and the new deployment of Microsoft Dynamics CRM must be the same base language.

NOTE: Redeployment between different base languages, for example from English to French, is not supported.

To bad that this snippet is only included in the course material for the Installation and Deployment exam and not in the Implementation Guide. The only solution was to do a full reinstall of Dynamics CRM with the same base language as the originating system and to do the reployment again.

Update
In the meantime I got new feedback from the support engineer (thank you José) who is responsible for my case. Microsoft will document this scenario either with a knowledgebase article or by adding the quoted note to the Implementation Guide.