+1 for adding true boolean search capabilities.
While Evernote's search grammar is already an essential feature that sets it apart from competitors, the inability to mix AND / OR is a bit frustrating and impedes the efficiency of using the app/service for organising and accessing large Note libraries (I have 10,000+ notes...!).
What I'd like is the ability to run/save searches like:
tag:2016FY (tag:expense OR tag:income) -tag:entered
Sure there are ways around that using temporary tags and/or notebooks, but this just adds an unnecessary (and clumsy) intermediate step that computers were built to do away with.
Like several of the other posters, I disagree with some of the arguments against adding this (user complexity, technical limitations etc.), although concede that the technical demands at scale are probably not insignificant. Complex boolean database searches can quickly have n+1 performance costs at scale.
As with many of these advanced feature requests though, perhaps the answer is to make them a feature only for premium customers?
Purely from a marketing point of view, delighting the small but vocal segment of power users is essential in maintaining an evangelical base for the product - especially given the significant bleed of 'casual' users to Apple's Notes app and MS OneNote, neither of which is as feature complete as EN.