Being in a local guild, I have to say that the hardest thing we come across is the people who don't know what a 1/4" seam is.
I helped teach a "Back to Basics" year round project for the guild. We started out with how to cut properly, then sew a true 1/4" seam.
From there, we went on to 12 different blocks. Starting with a large 4 patch. We even went into hand applique and machine applique.
Then did a couple of free motion/stitch in the ditch classes.
We had many of the members say they really learned lots from it.
I guess I feel that cutting properly and finding and keeping the 1/4" seams are the most important.
You are opening a whole can or worms here with that question. It will be interesting to see the answers to it.