Yesterday Mojang released a preview of a very significant change coming to Minecraft version 1.8: World customization. While players can already specify (and share) seeds, modify basic map features, and of course terraform manually the new world customization options will offer access to a broad range of variables to fine tune a newly generated world to suit their needs.
That said, it will be modders and map-makers in particular who will get the most out of the new terrain modification system. Tailoring a map to suit your needs when you're developing an adventure/challenge-oriented experience to share with others currently involves a lot of painstaking work, literally done one block at a time. These new tools will streamline that process so custom mapmaking will be a little less arduous and a lot more fun.