It's not really the horrifying place of taboo subjects it once was. Anyone with any literary talent left long ago, so there isn't really any epic copypasta. Copypastas are wonderful things the first week of life they have, then they become absolute trash, then they have sentimental value when you look back on them in an archive on ED or something like that.
The images you will see will either be: A. Old images that have been posted ad-nauseam due to their own sheer popularity, but someone keeps discovering them everyday. B. Random porn or gore, this is usually new stuff, but nothing provocative. C. Underage retards who shouldn't be browsing the site in the first place, adding pictures of themselves and asking if they are hot.
This wasn't always the case, but 4chan is far more mainstream than it used to be.
Pedophiles usually post whatever messed up crap they have on Sundays, avoid Sundays. Though most of them have been banned, IRL or OL.
You'll be enamoured with /b/ for the first few months you visit 4chan, if you take a liking to the place. After that though, you'll migrate toward other boards to look at stuff you care about.
The mods are almost never on, or are not really doing their jobs.
Desensitization occurs very quickly if you lurk on /gif/ for a few days.
Trolling is only possible if you post a thread on religion or politics, just like in real life.
EDIT: This is on /b/. You'll have to learn the nuances of trolling the other boards by lurking, which basically how you learn everything else about 4chan.
Epic really only happens when the life of someone is ruined, keep this in mind. Everything else requires creativity and humor, something most people on 4chan lack. Make sure you're not really morally compromised about such ruination.
Everyday is repost day.