Extremely important: having an additional passphrase (BIP 38)!
If your paper wallet isn't passphrase-protected, the entire contents of the wallet are up for grabs by anyone who finds it. It's like compressing all your coins into a piece of paper, open for anyone to steal. Very dangerous (spouses, siblings, Bitcoin ATM shop staff, OTC contacts, don't always have the best intentions...).
This is definitely advice that everyone should follow, but for the average investor in Bitcoin or altcoins, such things are simply too much because they go in the direction of trusting CEXs like they trust banks. Even those who additionally secure their backup with a passphrase sometimes store seed+passphrase together, which of course makes no sense, unless they think that someone will succeed in hacking their seed because it is
very easy to guess 12 or maybe 24 words out of 2048 available and then a passphrase will serve as additional protection.
I can only say that the world is full of people who look very nice and harmless, but unfortunately they often have very bad intentions when they sense an opportunity to get easy money.