Differentiating between ESB implementations

Most ESB implementations provide all of the services that are used to define an ESB, so it is hard to differentiate ESB implementations based on features. A better way to differentiate between them is to use the following four measures.

Supported deployment / runtime environments
Many ESB solutions are designed to be deployed into application servers, other heavy weight containers, or proprietary runtime environments. These types of ESB solution is ideal for distributed computing. They also contribute to vendor lock-in.
Ideally, an ESB solution should have flexible deployment requirements so that it can be distributed through out an enterprise.

Container/component model
Does the ESB solution use a standardized container model, such as J2EE, JBI, or OSGi, for managing deployed services? Or does it use a proprietary model?
Ideally, an ESB solution should use a standardized container model. Standard models ensure maximum compatibility and lessen the learning curve needed for adoption.

Coupling to other infrastructure components
ESB solutions often leave out infrastructure components like orchestration engines and advanced transports like CORBA. Instead they rely on plug-ins or other components to provided the functionality. Many ESB solutions require a tight coupling between the ESB and the added components. This means that you are limited to only using the added components supplied by the ESB vendor or must learn complex APIs to extend the ESB yourself.
Ideally, an ESB solution would provide a loose coupling or provide a standardized interface between the ESB and any added components. This allows the ESB to be extended easily and in a flexible manner.

Dependencies
ESB solutions have a lot of moving parts and complex dependencies. Some ESB solutions handle these dependencies by locking themselves into using proprietary solutions for things like security or JMS implementations. Others rely on standardized implementations as much as possible.
Ideally, an ESB solution would only depend on widely available standardized libraries to make dependencies easy to manage.

-Fuse ESB Getting Started with Fuse ESB Version 4.3.1