Host Startup Failure

In my experience, the failure to start a host instance is most often caused by an error in the BizTalk config file btsntsvc.exe.config. This can be because it is not well-formed XML or the content is incorrect.
For example recently I had to investigate an issue with automated unit test failures which turned out to be caused by an incorrect config file update. A WCF client endpoint had been added outside of the element. The error I saw in the event log was:

A failure occurred when executing a Windows service request.

Service request: Start

BizTalk host name: BizTalkServerApplication
Windows service name: BTSSvc$BizTalkServerApplication

Additional error information:
Error code: 0xc0c0153a
Error source: BizTalk Server 2009
Error description: A BizTalk subservice has failed while executing a service request.

Subservice: Tracking
Service request: Start

Additional error information:
Error code: 0x80131534
Error source: System.Data
Error description: The type initializer for 'System.Data.SqlClient.SqlConnection' threw an exception.