The thing you say it render as is a hyphen, not a minus sign. A minus sign looks quite different from a hyphen. Thus:

The other typo is that it should say \oint rather than \oinit.

"very thick" rather than "very think" was intended. The other issue is that the subscript C is far to the right of where it should be, as seen by looking at the other integral sign.

In some cases the spacing should depend on the context. The ordinary plus sign is a binary operator, and when one types "5+3", the spacing to the right of "+" is conspicuously larger than what it is when one types "+3", using "+" as a unary operator. Similarly in a\ln b and a\ln(b), the spacing to the right is different: much smaller in the latter.

OK I now see that \koppa and \stigma work when I'm not logged in, but not when I am. On my preferences page under "Math" I've chosen "MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools)". I would not think that any choice of preferences should cause these letters not to be visible.

I just looked at that page and \koppa and \stigma are working now. Was something done to fix them?