Advice for Intercampus Transfers (UC to UC)

What is the most important factor to consider when trying to transfer to another UC? Some people have told me your GPA, while others have told me its the reason for wanting to transfer. Also, how important are extracurriculars? In some other threads I’ve seen they’re not even discussed, only GPA, Prereqs, major, etc.
I’ve looked at my school’s (UCM) requirements to request a letter of reciprocity and I should definitely have it by the time I transfer, and all my prereqs should be completed as well.