ISO/IEC 29500 Standard

The standard is free for all to use and extremely well documented.   Microsoft has also included Open XML in its Open Specification Promise.

Interoperability

The ability to save, load and use the document format in a wide variety of applications and tools provides a high degree of document interoperability.    These many tools, both commercial and custom, enable developers and end users to interact with documents in a dynamic way offering a variety of new end user experiences and outcomes.

Backward Compatibility

The ability to convert binary documents to Open XML with a high degree of fidelity enables end users   to convert their document archives to Open XML, and then programmatically access the converted documents.   This creates, for example, an opportunity for developers to implement a custom document crawler or indexer, meeting the needs of end users in ways that a standardized crawler cannot.

Programmability

To work with Open XML, all you need is a library that can open and save zip files and an XML parser/processor.   ZIP and XML libraries are “server-hardened” with predictable performance and memory usage profiles enabling the ability to build Open XML applications that run server-side (and of course client-side).  The Open XML SDK makes it easy to build .NET applications that generate or access Open XML documents.

Integration of Business Data

Separation of semantic data from presentation markup is a fundamental element of good document format design.  Custom XML parts enable document solution designers to include business data directly in documents.  Content controls enable the formation of relationships between content and business data.  Content controls also enable delineation of content and specification of metadata about the delineated content.

More Secure

DOCX files cannot contain macros, and therefore helps eliminate an entire category of security threats.  If a DOCX document does contain macros, Office 2007 and Office 2010 will refuse to open it.

Smaller File Size

This particular benefit is important to people who manage all varieties of document stores, including Exchange servers, SharePoint, and of course network file storage.