Some of the differences between Ant 1.x and Myrmidon:

• Groups of tasks can be assembled into self-describing antlib task libraries. This makes it very easy to distribute and install tasks and data-types.
• A project can import other projects, and reference their targets.
• Properties and data type references now share the same namespace.
• Changes to task and data-type API:
  • An interface based API, through which a task interacts with the task engine, and the services it provides.
  • A well-defined object lifecycle.
  • Polymorphic types are supported when objects are configured.
  • Reference handling is handled automatically when objects are configured.
  • Aspects can be used to decorate tasks, to add facilities like logging, error handling, or user preferences.
• A set of well-defined services and APIs have been introduced, to allow tasks to do their work without depending on other tasks:
  • Execute external commands.
  • Execute other tasks.
  • Manage ClassLoaders and Class-paths.
  • Virtual file system (in progress).
  • Execute Java applications (in progress).
  • ...

There are plenty more features planned. You can read about them here.