Here's the first time we discussed QDF [webmasterworld.com]. It's become quite an evolved phenomenon since 2007 when the terminology first hit the press. For example, we can see some backlinks start out powerful and rapidly lose strength, whereas other backlinks start out as lightweights and grow over time.
Blended Search is another name related to Universal Search - you may be more familiar with that terminology. The idea, especially for short query terms, is that the user's intention as originally typed is often ambiguous. So what set of results will truly satisfy most people who type in a given query is under constant experimentation.
Commonly people think of Universal Search as the possibility of blending results from Images, Video, News, Local, Products and other verticals into one SERP. But blended search has even more going on than that.
For example, the same spelling (or misspelling) of a term can be used in many different meanings - just think of [apple] or [windows], for example. And Google sometimes forces a result onto page 1 that is taken from some more diverse user intention cluster -- even though that url would not normally rank so high at all on a pure apples-to-apples basis.
"Fresh" results can also be forced into some SERPs, as well as results from various informational, transactional, or navigational intention clusters. A strong example would be Wikipedia results, which often show up for a query when Google feels some searchers may have informational intention and others may have a transactional intention. Yes, Wikipedia has some great backlinks - but that alone doesn't account for how often it shows up in basic searches, in my opinion.
As a side track, there was a somewhat revealing bug a few years ago, when Google was evolving the technology to force certain results into certain positions (see the Position #6 'Penalty' [webmasterworld.com])
For some queries, the rankings on page 1 may still be a conventional horse race within one basic relevance algorithm. But that situation is becoming more and more rare. What we see more often today often involves some degree of "query revision". Essentially, Google works to read the mind of the search user and give them some results that they might have had in mind, rather than the straight results for what the user literally typed into the search box.
[edited by: tedster at 8:10 pm (utc) on Aug. 30, 2009]