It is highly recommended to play Borderlands 2 before the Pre-Sequel. Even though it is a prequel, the framing story takes place after the events of the second game and contains significant spoilers.

The main story has "hard" level sets for different areas. If you rush the main plot, you will quickly find yourself underleveled against bosses like RK5 or the Sentinel.

Quest items and NPCs are often hidden much higher or lower than they appear on the minimap due to the moon's vertical design; always look for jump pads nearby. Playable Vault Hunters