Many enemies that are in the backline are very fast and can cause your team to have a high stress count if you don't kill them fast enough.

Having at least 1 backliner that pulls or a way to deal with the opposing backline.

What you do with these frontliners is also very fluid: You can have a team centered around buffing a Leper so that his low ACC isn't much of an issue, or a build centered around stunning the entire enemy team with a frontline Hellion and a backline Plague Doctor. Generally, however, this will be either a Hellion, Highwayman, or Leper, due to their very high damage output. This one is a little bit more fluid, as you can have many different classes take up this role.

Having at least 1 dedicated frontliner that deals damage to enemies'.

This doesn't mean your party needs a healer, however it is highly recommended you take one for regular parties. The preference on taking which one depends on the person, as Occultist has more utility while Vestals have a higher chance to actually recover the health of your party. This will usually be a Vestal or an Occultist, as their backline capabilities are greater than that of a Crusader or Leper, and their heals can recover the health of your party by a much higher amount.

Having at least 1 dedicated backline that heals the party.

There are 3 different factors that usually make up a good team: This page is dedicated to party compositions that work well in Darkest Dungeon.

Darkest dungeon beginner teams