The tag mathematical-content seems too broad to me; but it seems we found agreement on this in comments.
For a tag mathematical-object: the tags seems like a bit of a 'meta tag' to me in that it rather describes the type (than the subject) of the question. However, a reasonable argument can be made that it actually does describe the subject.
Still, I do not fully understand the intended scope. Put differently, what is (and is not!) a mathematical object?
If we want a tag with this name it ought to be clear what this means (for the scope of the tag).
Going through the example questions, I would say to me only Z/nZ is clearly a (family) of mathematical object, as some set with some structure.
Already for the Lebesgue integral this is a lot less clear. Right, one could think of it (them?) as functionals on spaces of functions and so on, but still I feel that a certain flavor of integral is not among the things I think of when somebody says mathematical object. Rather this question is motivation for definitions
For showing that R is uncountable it is even less clear why this is to be tagged mathematical-object.. Sure it is about the mathematical object R but then just one aspect of it, and many a question would be about some aspect of some mathematical object.
Rather this question is about proofs in my opinion. Different ways to prove one and the same thing are to be compared.
Finally I am really at a loss for Eisenstein criterion. This is a result or an algorithm if one wants. Now, in theory, yes this can also be considered as a mathematical object, but frankly if every result and every algorithm is a mathematical object then we are back to mathematical content. Or, is the object here the polynomials that are concerned. But then again 'everywhere' some object will be concerned.
In brief, given the examples I am sorry but I do not understand what should be tagged mathematical-object.
There is the feature of tag wikis. I propose (here and in general) if somebody proposes a tag they should in addition propose a text for a (short) tag wiki that explains how to use the tag correctly.
To sum it up: I do not understand the intent of the tag and would not know how to use it. Thus, at the moment I am against it. Please explain how exactly it should be used, keeping the issues I raised above in mind. Then I might likely change my mind.