Discipline
Most of the people while reading this word would think about the military discipline and never associate that with agile. It's quite funny that for many people being agile is mostly lacking discipline, while in my opinion it's impossible being successful in the agile world without discipline.
In the old days life in a software project was ruled by The Gantt, a simple diagram illustrating the start and end date of every activity in a project. Those dates were thought in the design phase of the project and defined by different people (usually the analysts) than the ones that were implementing the actual software. Developers were receiving a long and detailed frozen specification document from analysts and were then supposed to handover to a testing department the software they've realized.
Now let's think about the agile world as seen by someone used to work in the waterfall way: there are no separate phases (analysis, development, testing...) but just a small requirement gathering at the beginning of the project, maybe there aren't even strongly defined release dates, there's not even a project manager telling people what to do next: it sounds really awful! "How can those people work in that way? That's anarchy! They can just do whatever they want. How can they even know if they're on track?!?".
The Manifesto for Agile Software Development says that working software is more valuable than comprehensive documentation, so an agile project doesn't have any big specification document or such, instead there's a list of feature the project should provide. A team without discipline could then think that instead of realizing those features one at a time they could go on in the old way and start doing horizontal development: "let's now start building the persistence layer, then let's add a business layer on top of it" and so on, maybe even parallelizing those developments without any kind of integration until everything is almost done. This would make impossible showing anything to the customer until the end, because everything would be 90% done. How can that team be sure that they're developing what the customer really wants?
A disciplined team would do something different: estimating the complexity of all the features and then start working on one of those at a time, maybe the project's most important, and implement it completely (from GUI to persistence for example). The team would then show the implemented features to the customer as soon as possible, in order to get feedbacks that would allow them to know if they're moving in the right direction or not. Knowing how complex a feature is and how much time did it take to realize if the project is on track, is falling behind or even if it's going better than expected. Making the daily work visible, maybe using a burndown chart, will immediately show how the things are going, allowing the team to take actions if things are not proceeding so well, but it requires a strong discipline (keeping always up to date the scrum board or kanban board for example), even if it would be easy sometimes to cheat and say that something not done is done "because there's time tomorrow to finish it".