For what should we use the expression-manipulation tag? It doesn't have a wiki, though from the 13 questions it seems like it covers structural manipulation of expressions. While 13 questions are not too much, there is another side of the coin: structural manipulation is a very important part of Mathematica, and is underrepresented among the tags. We always fall back to use such low-level manipulation whenever regular evaluation is not enough, and we all like the beauty of the injector pattern and the T&S method.
The topic is only partly covered by evaluation and replacement, which are not directly about structural manipulation. Furthermore, there is the simplifying-expressions tag: it seems like that expression-manipulation is used whenever the other is used in a somewhat interchangeable fashion. Now I understand, that for example rearranging knowns and unknowns in an equation both involves algebraic and structural manipulation in Mathematica, but since there is no wiki for the latter, the appropriate tag lingers there without any meaning.
I would like to hear your opinion about this issue before submitting anything to the tag wiki, or before pushing for tag removal, retagging, synonymizing, or whatnot.