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.
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
From Martial Way
The final, most important thing to pick up is, "Is this a place I can see myself coming to a few times a week for the foreseeable future?" This is a general feeling - the total mix of the instructor, the other students, the thing being practiced, and the energy in the room.