I can't speak for the entire org, but when I was in Display Ads, promotion went to people who moved the needle on certain metrics, e.g processing latency, reven

I can't speak for the entire org, but when I was in Display Ads, promotion went to people who moved the needle on certain metrics, e.g processing latency, revenue, throughput, resource usage, etc. More often than not, impacts like that were accomplished by shipping new features (while deprecating legacy code) instead of fixing bugs. If you're going for promotion, having launched something that makes an impact (appropriate to the level you're going for) is a must. That said, it's a fairly common occurrence that once you get promoted, you move to a different team, leaving your old projects to your former teammates.