I think many people, no matter how long they've sewn, have trouble with -- or are afraid of -- Y seams.
I've just done a 6 pointed star quilt and I learned a lot, mostly from all the mistakes I made. Some Y seams don't scare me, but the ones in this quilt were daunting -- especially when you're trying to match points from stars above and below rows.
Might be too much for beginners, but simple Y seams, as in attic window quilts, would be good to teach, I think.