What do you actually track in your training log?

Some people log every set, rep, and rest period. Others write a sentence about how the session felt and call it good.

Before you start your own journal here, we're curious what's worth tracking for you. Weight and reps, bodyweight, sleep, food, mood, or notes on how a lift felt?

And if you've kept a log for a long time, what's the most useful thing it's taught you?

What do you track, and what do you skip?