I'll start off with one that I spotted the other day while researching: Email notification length vs. CTR. That's one I never considered before outright, yet

I'll start off with one that I spotted the other day while researching: Email notification length vs. CTR. That's one I never considered before outright, yet I've crafted all my notifications with a sense for that metric. You want your notifications to be just long enough to be tempting, but not so long that they can simply read the email and never click through.