As I see it, the difference is that we now have capable game engines freely available. Indie studios can, for the most part, offer the same quality of gameplay. AAA studios can only really differentiate themselves by how much content they shove into a game.
In particular, this also somewhat limits creativity of AAA games. In order to shove tons of content into there, the player character has to be a human, the gameplay has to involve an open world, there has to be a quest system etc…