For me the biggest disappointment is when they turned off the ability to search for "an exact match within quotes", as well as the ability to use +plus and -minus to refine results. (EDIT: could be wrong about that, I thought it was disabled at some point)
But the most dumbfounding is the fact that you still get several year old results whenever you search for a bug, hardware/driver issue, or even simple programming questions related to a not too old framework or API. Every time I have to go into the advanced options to restrict the results to one year, or one month.. you'd think they would make recent activity the default and then let the user know they can "search older results".