One of the best things I like in Maven is that every maven project I touch has the same directory structure. I immediately know where to look for java sources, web resources, test files etc. 
On the contrary, I hate Ant-based projects, because virtually every such project uses its own directory structure. Every time I have to spend some time on understanding the layout of the sources.
Fortunately, recently I see that many Ant-based project, even if not using maven, adopt the maven standard directory layout, so it seams that this layout becomes slowly the de-facto standard. So why not apply this concept into Ant directly?
We can take the maven directory layout (even if it is not absolutely perfect,  introducing different one would be the worst thing we can do) and make it also default for Ant. This would mean, that for tasks like javac, jar, war, javadoc, one does no longer need to specify argument like srcdir, destdir itp (still can do it obviously to override default values)
See also http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html.