I've been laying warnings that this would be coming and you've seen me grouse about it for over a year, but now the time has come to really dive into Maven for Domino developers.
To lead into it, there are two main topics to cover: what Maven is and why you should bother.
What Maven Is
Maven is a build automation tool, primarily for Java applications but able to work with a number of other languages and environments.
The concept of a "build automation tool" is a strange one when you're coming from a Notes/Domino perspective, and it's the source of a lot of consternation when moving to it. In classic Notes, there conceptually was no build phase for an application: certain things would be compiled on save, but there was rarely any need to think about this. Designer was the way to write applications and it took care of it. With XPages came a bit of Eclipse-ism with the notion of "Build" being a separate, not-necessarily-automatic stage, but there still wasn't much user-facing configuration going on: other than maybe adding a source folder, the IDE just kind of took care of it.
Even for OSGi plugin developers, the need seems a little arcane. Eclipse does have project and build configurations, and it runs through build scripts internally when you export Jars or build an update site. Again, though, this is all largely hidden and the user doesn't normally have to think much about it.
Where this comes in, though, is when you want to start expanding your projects in ways beyond the "single bag of code" stage: automatically including pre-packaged dependencies, making the project cleanly available to others down stream, sharing configuration across projects, and, particularly, automating building, testing, and deployment with an environment like Jenkins. Maven (and the alternatives like Gradle) provide important structure and meta-information to do these things and scale them to ever-larger tasks.
Why You Should Bother
I've found the pro-Maven pitch to be kind of a weird one, since it's sort of like unit/integration testing in that, before you do it, it doesn't seem worth the hassle, but then, when you've switched over, it seems crazy to not do it. Like a cult, I guess, but a somewhat better idea.
It also makes testing that much more natural. Building and running automated tests is certainly possible in Eclipse, but Maven's structure strongly encourages it (newly-created projects start with JUnit and a tests directory, to nudge you in that direction), and having test running be a phase in building makes it much more foolproof. The virtue of writing tests creeps up on you: even if you don't go whole-hog TDD, getting into the habit of starting each bug fix with a failing-then-successful test case means you now have a bug you'll never have to see again. Granted, as with a lot of other aspects, the nature of Domino development creates some hurdles, but it's still worth it.
And finally: features, features, features. Once you get comfortable with Maven, it becomes much easier to spin up accompanying source and Javadoc packages for your projects, add in other languages, filter content during builds, create alternate build profiles for different situations, bring in remote resources easily, deploy to targets automatically, manage versioning cleanly, and so forth. These are all things that are possible in the absence of a system like Maven, but Maven brings them all together in a way that is understandable both by your IDE of choice and by faceless servers.
The next step will move from high-flown concepts to some brass tacks: preparing a Maven environment for Domino development and getting a look at what Maven's configuration files look like.