The idea of dependencies for WordPress plugins has been floating around for some time: #10190, #11308, #12612.
Although plugins can degrade gracefully by using actions and filters, there are some things they can’t do, due to the simple fact that they need to be activated first. Such as:
Activation prevention
Cascade deactivation
My hope is that this meta-plugin will be rolled into Core at some point. Until then, we can learn more rapidly what works and what doesn’t.
Go to the wp.org page to see how easy it is to define a dependency.