Not taking the "Fully Diluted Market Cap" metric into account seriously will cost a lot of new enthusiastic crypto investors money and desperation.
The main difference to the market cap (actual supply times price) is that a fully diluted market cap takes into account the maximum possible supply times price, i.e., also the coins that are not mined or minted yet. Many projects have really a fraction of the total supply in the circulation, which makes their price and market cap appealing as a good investment.
To see how big a difference the diluted market cap creates in practice, I have analyzed the top 100 coins by their market cap (to date 2. 4. 2021). From all the top 100 CMC coins I am reporting such coins whose circulation currently makes less than 75 % of the total supply. For these coins, I show the relative ratio between market cap and fully diluted market cap, and also the adjusted rank that supposes all the coins went immediately into circulation at the current price. The coins are ranked from the ones that have the smallest ratio of coins in circulation.