歯が欠ける - tooth is chipped; お金が欠けている - money is lacking
XXは面白みに欠ける/XXは説明能力に欠ける XX is lacking in 面白み/説明能力
君に欠けているもの - "the things you are lacking"
My confusion lies initially with how i would interpret XXは面白みに欠ける. This appears similar to XXは試験に受かる.
In this case, 試験 itself cannot "pass", only XX can "pass" in regards to the 試験.
Therefore: XX passed the test ~= XX passed, in regards to the test.
So: XXは面白みに欠ける - XX is lacking in 面白み/説明能力 ~= "XX is lacking in regards to 面白み"
According to the link, because "試験 itself cannot pass", 試験が受かる cannot be grammatical. This is where 欠ける is different. In 歯が欠ける, 歯 itself can be 欠ける'd.
So going back to 君に欠けているもの, i don't have the option to interpret it like XXは面白みに欠ける. In fact i feel like i can only read it as if it were "君が欠けているもの", just like 歯が欠ける.
金が欠けている - money is lacking
君(に/が)欠けているもの - the things you are lacking
In this case, what is the difference between 君に欠けているもの and 君が欠けているもの?