... and me of three years later with an additional note. (Possibly for me in the future.)
One particularly unusable scenario is Courier New at 8pt. Why is this even a thing? Because it's especially useful for making boxes-and-arrows diagrams in which the boxes represent text files, and actually contain a representation of source code or other plain text. Example: Diagram shows which file contribute to a software build process.
Courier New looks light weight at the best of times, but in Visio, at 8pt, the thin lines translate to light-grey fuzz. Could use bold, but that is too heavy and distracting, and precludes using bold selectively.
Would-be solution: Get a copy of the font "Dark Courier" (TTF), which apparently originated with HP. This seems to work well in, for example, Word. However, in Visio 2010, while it shows up on the font list, Visio doesn't want to actually apply it.
I loaded that font into FontCreator, and noted that there was one incomplete glyph, which I deleted. I then exported the font as an OTF. Not sure which trick made the difference, but now Visio can use Dark Courier, and it looks considerably better. Bold version also looks OK.
-- Graham