I mean the spartan user interface approach the followed in the early 2000s in their core product.

We give for granted now that applications should be clean, focus, etc, but for many years that was not the trend. To put it into perspective, its major rivals at the time (Yahoo, and MSN) followed the opposite path.

Google had a huge impact on how web interfaces are done today.

They also popularize ajax when they launched gmail but that its another story.