I really like Maven, and I really like the declarative build style, but recently I finally came to understand why Gradle is better.
For small projects that produce a common library JAR, you can still use Maven, but real-life, complex software projects always contain a lot of support scripts for deployment, copying artifacts, and so on. For some of those tasks you can find Maven plug-ins, for most of them you can write Maven plugins, but in real life you have shell scripts to do the job.