Reserve this for your less mechanically oriented games, though you can still do epic quests. It's a kind of game mode: Magical Unicorn Adventure.

Premise is that when starting the game the dm sets some parameters for the players to (optionally) consider when making characters. Then the players make their characters, except that all character creation rules are now arbitrary. Players may make their characters as weak or powerful or balanced as they want - and if the game sucks then it's all their fault. This means they can start at whatever level as any class (even made up), with any weapons and equipment and abilities.

You as the dm then run the campaign as normal.

I discovered that this House Rule is best reserved for players you trust. This removes the min-max potential from the campaign now that players can't abuse the book to make the most powerful starting player. To level up, just use the milestone method.