This isn't my answer, but it definitely resonates with me. It's from ThoughtWorks' Technology Radar from October 2012:
Two things have caused fatigue with XML-based build tools like Ant and Maven: too many angry pointy braces and the coarseness of plug-in architectures. While syntax issues can be dealt with through generation, plug-in architectures severely limit the ability for build tools to grow gracefully as projects become more complex. We have come to feel that plug-ins are the wrong level of abstraction, and prefer language-based tools like Gradle and Rake instead, because they offer finer-grained abstractions and more flexibility long term.