The concept of extension- the "unbendable arm"
Seated forward roll- took me several years to get that one.
Standing forward roll- got it basically right away.
Standing sit fall.
Etiquette for bowing in and out of the dojo, bowing on and off the mat, bowing to a partner.
What I didn't ever get told is what to call my teacher. I struggled with that for a year or so, till I relaxed into "Sensei", though I still call him by his given name sometimes.
Rules can be a pain in the butt, but it is nice to know what is polite right away.