One thing that would be nice for some of the projects we work on with clients is the ability to globally exclude artifacts.  For example, I would like always to exclude commons-logging and log4j because I am using slf4j and logback instead.  Many third-party artifacts we use depend on one or both of these, and it will be difficult to add an <exclusions> section to each artifact that transitively includes commons-logging.  This feature would most likely need large warning signs that it should not be used except in extreme circumstances and only if you really know what you're doing, as it could lead to odd behavior and broken builds.