I'd say the scarier thing about 121 is the legal consequences of things. I worry more about getting violated for something way more than I do about actually getting hurt, because realistically you should never be in a situation where that's a possibility.
I'd echo what
@Screaming_Emu said. Be wary of short cuts or doing/not doing something for the sake of convenience. There's been a couple times where getting lazy/complacent has led me to stress or kick myself over something for days afterward where if I had just taken the extra 5 minutes I wouldn't have given it a second thought after.
The other thing I'd add is fight to not get complacent every chance you get. It's easy to do especially when everything is so regimented like it is in the airline world. Hell, I have every normal checklist we used memorized verbatim, but always force yourself to take the time and do things the correct way. If you get complacent you WILL miss something at some point, and it's painful flying with really senior people that seem "too cool to care" and slack off.
Another thing... breathe. This career can be stressful at times and it's easy to let your emotions or temper get the better of you, especially if you're dealing with what is sometimes downright incompetence. Anytime I get into a frustrating situation I try to take a breath, be firm but calm, and ask myself "Is this worth getting worked up over? Will I still care about this in a week?" 99% of the time the answer is no, and down the road it will keep you from becoming "that guy" that nobody wants to work with.