I can sympathise with both sides of this - theducks is quite right in saying that of all the bits of geek that exist in calibre the search bar is probably one of the simplest.
To be fair it does still have its challenges though with the various permutations of choosing to use or not use quotes & equals signs for instance, tri-state boolean fields (:false/:true vs :yes/:no) or getting funky with regular expressions. You also have to remember exactly what the field was called (e.g. published or pubdate?), and remember to put the # in front of a custom column name. If you don't use search syntax regularly it is understandable to want to avoid having to learn it at all, which is why the tag browser is such a great feature.
So I'm not overly against trying to help users keep their hands on the mouse with a simple menu item click. I'm just not sure though how useful the menu options would be and what they would consist of? For instance do they only need to represent the negative cases? i.e. no tags, no pubdate, no comments etc. Which fields does it need to offer options for - all of them? And what about custom columns - should the plugin try to create menu items for those too?
Sometimes it is easier just to leave it to the user to type - or keep crib sheets as suggested by theducks
