4. Downloadable content
It all began with the infamous Horse Armor DLC, the first downloadable add-on for 2006's The Elder Scrolls IV: Oblivion. This delightful pack, as you may recall, promised to protect your horse from danger for a paltry fee of $2.50. But really, it was just a cosmetic upgrade, and a lot of gamers felt insulted by such a blatant attempt at prying the cash from their wallets. Thankfully, DLC has gotten a bit better since.
There's now a pretty clear distinction between cosmetic microtransactions and mini-expansions, even if they all technically fall under the DLC moniker, and we're seeing more and more instances of fantastic content add-ons. Undead Nightmare presented a unique, zombie-filled twist on Rockstar's Red Dead Redemption; Minerva's Den sent us back to BioShock 2's Rapture to experience an intelligent self-contained narrative; all the DLC packs for Fallout 3 and New Vegas kept us wandering the Wasteland far longer than we ever would've anticipated. There are, of course, many more examples of fantastic DLC, and though content add-ons will continue into the future, we'll always remember when it all began.