Many customers are hitting a new bug in ESX preventing them from powering on virtual machines or VMotion them.  There is a published knowledge base article on this which reads>>
Unable to Power On virtual machine with “A General System error occurred: Internal error”
A virtual machine fails to power on, fails to leave suspend mode, or [...]

Here is another new and very useful article from our knowledgebase discussing the majority of reasons for VMotion errors. The online version can be found here: Overview of migration compatibility error messages.

A common problem I see is when VMotion fails at the 10% mark with a timeout. You might see a dialog popup which says-
Operation timed out
Tasks: A general system error occurred:
Failed waiting for data. Error 16. Invalid argument
The VMware Knowledgebase writers have assembled a great list of possible reasons for this problem and I [...]