Despite the major version bump, this should be a drop in replacement for almost every project, The two major possible incompatibilities are:

• The minimum supported Java version is now 1.5, up from 1.4 in 1.2.0 and 1.3 in 1.1.6.
• dom4j, XOM, and JDOM are now optional dependencies so if a project
  was depending on them to be loaded transitively it will need to add
  explicit dependencies to build.

On the plus side, this makes classpaths much smaller and less complex, and will shut up a lot of noise from static analysis tools.

In addition the Jaxen core code is also a little smaller and has fixed a few minor bugs in XPath evaluation, mostly weird cases found by fuzzing that are unlikely to arise in practice.

Deprecate pattern package.
Require Java 1.4.
Don't bundle org.w3c.dom