Google released notes for improvements made in November and December--including 30 tweaks that happened in December. Some of the announcements were never posted before this posting on "Inside Search" 30 search quality highlights (with codenames!): December.
One noteworthy update for SEO professionals included a featured dubbed "Megasitelinks" which allows for sitelinks to be displayed relevant to location. This algorithmic change localizes which sitelinks will be displayed to a user when searching for a brand website. Google has been making a decent number of changes to its Sitelinks in recent months. In some cases, the Sitelinks have been expanded to include Google+ data.
Within it's December search update list, another curious item was some advancements to image search. Google posted two changes to the image search system:
After doing a very quick search it became apparent that spam detection (and enhanced malware detection) are becoming increasing vital elements for search engine companies. The web is not always a safe place to browse, which was demonstrated in a quick search by us today as our first search result returned a malware website within Google Images.
Google focused on improving Byline dates, and now byline dates are more trustworthy thanks to the work in December. In the past, old articles would be shown with recent byline dates, which was down-right confusing. Google says it’s improved what it calls “soft 404″ detection, which should help a lot.