In which types of search do Google, Wolfram Alpha, Bing and Quora most clearly fail?
The obvious answer would seem to be natural language queries, such as questions. Apparently, the Google Knowledge Graph has been restructured to better handle natural language. Wolfram Alpha defintely handles SOME natural language queries. Bing results are clearly inferior to Google. Quora internal or native search is much worse than Google search of Quora.
One thing that surprised me about Question Answering Systems, such as Apple Siri, was that the search is so good that it could tell you how to say almost anything in most European languages without Machine Translation, in other words simply based on search and the availability of text online.