What are your thoughts on the current way promotion boards are done?

Promotion Boards recommend you to the next RANK not the pay grade. The Board members are not basing the Soldier's readiness for promotion off of their ability to memorize facts, stats, and quote regulation. They are looking how can the Soldier articulate their knowledge. Your leadership at the Platoon Level/Squad/Section level recommend you based on your overall performance at your MOS and potential to serve at the next level of responsibility. Knowledge is power, and by knowing the publications the Army uses you are demonstrating your ability to research, retain, and demonstrate that knowledge. Every MOS has a different set of core technical/tactical skills you need; but as a NCO regardless of MOS you must have the foundation on the Army's Doctrine to instill the same on your Soldiers.

SGT (Join to see), great question. The job of ensuring that core MOS competencies are trained and evaluated is within the Platoon structure. The PSG, along with training guidance from the PL, trains and evaluates their personnel to ensure that they can perform their MOS related tasks. When that packet comes across my desk as a 1SG, I have confidence that the PSG has done their job and that SPC is ready to be a SGT. When that SPC enters the promotion board, the board members are evaluating them on a different set of skills. Appearance, demeanor, confidence, overall knowledge, etc. These are also important characteristics that NCOs should possess.

Leaders will not send you to a promotion board if they felt core MOS competencies were not being met. If a board focused on core MOS competencies, then that promotion board should be made up of peers in your MOS. How will that work for MOS's will low numbers? I'm a flute player for the Army Band. Do I need to perform a recital to get promoted?

Promotion boards for SPC should be focused on regulations. This is the stepping stones for getting familar with Army policies. I get it... The Army is huge and there's lots of information that wont apply to you, but a Soldier needs to be able to read and understand regulations. This becomes important for roles they will assume later in their careers. They will need to be the ones leading Soldiers in their daily task.
Promotion boards for SGT's should be focused on regulations, leadership ability, and how well they take care of Soldiers.