Your crew needs to bring specific equipment and weapons to make stealth or a gunfight easier. You also need to pay attention to your crew's health and ammo.
Examples of game-tied DLC (paid downloadable content) are masks, weapons, a skill set and locked achievements of games such as Hotline Miami Deluxe Edition and Speedrunners. This means you can't fully unlock all of the content available for Payday 2 unless you purchase these games as well.
With Crimefest 2015, the developer (Overkill) decided it would be a good idea to add stat boosting gun skins which can only be acquired through use of real money. This makes for a pay-to-win scenario where people who use real money can gain advantage over non-paying players.