Why find things that are ossur to do on Shabbos not because they fall into the category of any particular "malacha" (type of work) but rather because of "uvdah d'chol". Something that is normally done during the week and not on Shabbos. While this explanation seems lacking this is precisely what I am asking. What is the "issur" "uv'dah d'chol", why is it ossur? Does it have something to do with maris ayin (because it "looks" like you are doing a malacha?) Is it a "gezera" (some sort of decree) lest one come to do an actual malacha? What is "uvdah d'chol" and why is it ossur (cite sources, earlier the better if possible.)
From the Teshuvas HaRambam (106) this issur is to prevent going too far in what you are doing and it leading to doing an Issur DiOraisa on Shabbos.
Based on Tosfos Shabbos 126b ומדבריהן Shulchan Aruch HaRav (O.C. 106:18 (from the Magen Avraham) learns that there is an Issur of doing weekday-looking work as this denigrates Shabbos. In other words, Tosfos establishes two problems - one, it will cause you to do an Issur - this cannot be dispensed with for the sake of a Mitzvah, and the other is just a general denigration, which can be dispensed with for the sake of a Mitzvah.
I think the denigrating Shabbos issue means it would cause people to see the action being done and to take Shabbos less seriously and come to violate an Issur. Tosfos on Beitza 29b says that if there isn't a concern that people would see (in the whole potential action, not just this circumstance) then this latter problem is not a concern.