Easy to learn hard to master

This is one of the famous game designer phrases that gets slammed right in your face if you put your nose into the gaming industry.
It is primarily referring  to game mechanics, but also can be put in relation to the overall game experience. Since your overall game challenge curve should also rise from the bottom up.

For me good games have game mechanics with depth but almost none intial confronting complexity therefore they are easy to pick up.

Example Street fighters:

Depth Layer 1: Hands on! You can just start playing the game by mashing some buttons, you don’t have to understand what a single button does and immiediet can start bash the shit out of opponents.

Depth Layer 2: The basics! You will get an advantage by learning what each button does and getting a feeling for the timing and the range of your moves.

Depth Layer 3: Mastery! Due the exact timed button combinations you can fire off powerful combos and transform yourself into a wild tornado of destruction, which rewards your time investment into the game.

Time Well Spend!
Beeing able to become better at a game, or to accumulate more virtual goods is an important key motivator for long term player engagement.
You want to be able to show off your time spent (very phylsophical).