Big changes I can think of in the last few years that we've publicly announced:
* The index is several orders of magnitude larger.
* Most documents that you see in results can now go from crawl to serving in a matter of minutes.
* Ranking for the extreme head is pretty much SEO free.
* Improved ranking for long tail documents with little data available about them.
Those are just the really big launches I can remember off the top of my head.
Most of our day to day work in general falls into two categories.
1. Making our existing systems faster, fresher, higher quality. 2. Searching for brand new sources of data, most of which don't work out that well (like in any other research area.) and occasionally finding the diamond in the rough that dramatically changes search results enough for at least the SEO community to notice.