When asking a question on Super User, add a suggestion for users to include their "operating system" when adding tags.
It's such a common omission in questions on Super User, I think an extra reminder would be justified.
the site is about trusting users as well, if we start "suggesting" to include the operating system tag, later people will want to include a hardware specific tag, and later start demanding stuff.
IMHO, if you believe a operating system is relevant to the questions, would be enough to add a comment politely asking the user to add one, or someone with enough rep. This kind of participation is what make users learn and improve their questions.