AIKIDO HIBIKI
What exactly is being developed?
Most people come to aikido seeking some kind of personal development as a priority. Some criteria are easy to imagine ahead of time, however vague, like patience, perseverance, and physical fitness. Other benefits may be difficult to imagine.
From Tradition
Many Japanese traditions are practiced and passed on to the next generation through forms. Forms serve as "instruction manuals" as well as "containers". By the process of learning and mastering forms, one develops the ability to see the external (the teacher, the example) and the self (what one oneself is doing). One also has the opportunity to see the internal (what is happening inside one's mind). One may do the forms in a robotic or superficial way, or in a way that is "alive". This is akin to following the "instruction manual" literally and only the "letter", not the "spirit". In seeking to do the forms in an "alive" way, one must not only see inside oneself but also begin to manage it.
In aikido, the forms are almost always done with another person. Because of this, there is the ever-present task of adapting to the individual partner. Again, this is in the context of minding how much forcefulness and roughness one is using - in aikido, we emphasize reducing these as opposed to caring only about accomplishing the task. In addition to that ever-present task, there is the moment-to-moment communication with the partner - that is, one is not just grasping the partner as an object, but striving to notice and move in harmony with the partner's energy. Despite the forms being prescribed, we are basically always doing them imperfectly. We practice to notice and adjust to the moment-to-moment situation as it is.
From Martial Way
Of course, we may consider "noticing", "adapting", and "communication" in a very open-ended way. In aikido, we also have the framework of martial context. In a mechanical sense, we could call this an emphasis on efficiency and functionality. Further still, at some point, we also emphasize meaning, such as safe/dangerous, advantageous/disadvantageous, optimizing/weakening partner's power, etc. When this martial view merges with the above "forms view", we not only consider, does this position/movement/timing, etc. make sense in the context of the next thing that happens, which concerns safety, advantage, optimizing/weakening power, etc.
Connecting all this back to "what is being developed", a student is basically polishing their sense and sensibilities. We are constantly trying to attend to more than just accomplishing the task at hand - how and who we are as we do that task.