What really distinguishes, the Church, from making a Feast from Solemnity. I realize ranking of importance, but is there any other criteria?
Solemnities are of greatest importance and begin celebration on the prior day’s evening with evening prayer and some have a Vigil Mass. Sundays and Holy Days of Obligation are Solemnities. Feasts are secondary importance and do not have a first vespers or Vigil Mass, except those of our Lord on Sundays of ordinary time and Christmas time.
The solemnities focus on key theological truths, e.g., Sunday celebrates the day of Resurrection.