We all know that BizTalk is an commercial product from Microsoft for ESB (Enterprise Service Bus).
What are other ready-made features which BizTalk supports over the AppFabric Service bus. Or in other words what does AppFabric Service Bus doesn't provide to the features of ESB ...?
Is the term "Service Bus" misleading as far as the AppFabric Service Bus is concerned with respect to Enterprise Service Bus...?
The BizTalk and AppFabric Service Bus teams at Microsoft are actually tightly collaborating and we (I work there) are integrating BizTalk's capabilities with AppFabric and Service Bus over time. For BizTalk we have Windows Azure AppFabric Connect - which lets you plug BizTalk into Service Bus. At PDC'10 we announced (http://bit.ly/9gm2zI) that we'll be bringing a subset of the BizTalk integration features into Service Bus - you'll be hearing a lot more about the extent of those capabilities over the course of the current year.
For now, think of the products as highly complementary.
I hope that helps.