This was listed as one of the updated in SP1, and was actually released in the December CU. So what has really changed?
For all the default Metadata Categories in FAST for SharePoint the default behavior is to map auto-discovered crawled properties to the full-text index.
This leads to many irrelevant fields being searchable. The update is in fact adding many of these fields to an ignore list, setting the IsMappedToContents property of the crawled property to false.
The full list of the added ignored properties can be found in the function ImproveDefaultSchema in C:\FASTSearch\installer\scripts\include\commontasks.ps1.
The relevancy improvement is to avoid garbage into the index.
Thanks to Microsoft for providing this information.