It seems that the design elements were built progressively, while adding new features, by getting user feedback on what works and what did not. That was partly the reason why "Google Labs" existed in the first place.

I think there was a clear line drawn at least with the G+ integration, since they started consolidating the appearance of all services under the same UI and same color codes. There is a now a clear "design vision" in Google (even if it clearly sucks bad) and it's clearly not as Engineers-driven/Fonctionality-driven as before. Having Icons without text next to it is not just a style issue: it's a well known flaw in terms of usability. Everyone knows icons+text are superior to icons only. Especially when your icons are dark grey on light grey background. No differentiation whatsoever. I can't believe someone was actually paid to do this.