I'll add my request for this feature; I was in the middle of asking the same question when the search brought me here.
I would suggest that the ability to "Quick Add" or "Quick Ignore" a tag should be available on every question page as well as on the tag's page.
On a question page, I would suggest that the tags listed for that question include two buttons similar to the X button next to tags in your interesting or ignored list on the main page.
One button would have a + symbol rather than X with the hover background being blue or green rather than red. Alt text for the button would read "Mark Tag Interesting" or something similar. Clicking on that button would add the tag to the interesting list.
The other button would look like the canonical Do Not Enter sign: a red circle background with a white - symbol. No hover background change, but the alt text would read "Ignore This Tag" or something similar. Clicking on that button would add the tag to the ignored list.