The renderer and tooling were heavily overhauled, though it's hard to say if that's a "Source 2" thing. Engines are worked on constantly, systems are constantly

The renderer and tooling were heavily overhauled, though it's hard to say if that's a "Source 2" thing. Engines are worked on constantly, systems are constantly being modified and features are added. Deciding to add a 2 at the end is mostly a marketing decision, since gamers fall for that hard. I worked on an engine that, despite multiple huge overhauls and tech improvements motivated by our games, people suggested it was "the same buggy engine" as the first game we released since we didn't change the name for our in house toolset.