A common, perhaps dominant, practice of agile methods is to develop a list of features (often called stories) for the software that's being built. These features are tracked with index cards, work queues, burndown charts, backlogs, or whatever your tool of choice is.
On the whole I like this kind of approach. By breaking down everything you need to do into small tasks that you can complete in a week or few, you can visualize progress and get a sense of how much you can get done. I've often said that the key benefit of iterative development is to reduce risk by forcing completion of software in chunks instead of the waterfall habit of leaving long and hard to manage activities (testing, integration) till late in the project.
The problem comes when this list suddenly grows horns and fangs and becomes a Fixed-Price Fixed-Scope Big Up-Front Project Plan. Craig Larman once joked that the waterfall process has strong antibodies that reject iterative processes by warping them into some form of waterfall. RUP has been a common victim of these antibodies, seeing its phases turn into some variant of the analysis-design-build-test conveyor.
The key to beating off the waterfall is to realize that, as Dan puts it, agilists value Outcomes over Features. The feature list is a valuable tool, but it's a means not an end. What really matters is the overall outcome, which I think of as value to the customers.
An important part of this thinking is that you expect the feature list to change as the project goes on. This happens you discover new things that you can do, and re-prioritize old things. This is the essence of adaptive planning, which has always been a key indicator of agile thinking. This results a big shift in how people think about a plan. In plan-driven projects, success and failure is often worded in terms of "did things go according to the plan?" In agile projects this is a meaningless question, because plans change so often. The plan is a tool, primarily one that you use to gauge the effect of changes: "how will adding this feature affect what we do". The plan is a tool to figure out what should fit in the FivePoundBag. If your plan's not constantly changing, you are very unlikely to be doing adaptive planning, and hence aren't agile.
Feature lists have another problem - you easily lose sight of the context that makes the feature valuable. This is a reason why Alistair Cockburn is a proponent of use cases, because they concentrate on a narrative of how someone uses a system. Marc NcNeil also talks about this in terms of Customer Journeys. The weakness of use cases in planning is that they don't give you clear units to tick off so you can assess progress and project consequences of choices into the future. That makes them less useful as a planning tool, but that doesn't negate their value as tool for imagining what a good outcome would be.