How to gather user opinions via a survey based on tagging?
You can easily do this with taxonomy free tagging

Step 1 : Create a vocab, going at admin/structure/taxonomy/add
check the screen shot after adding a vocabulary named as survey.


Step 2: Create custom content type,going at admin/structure/types/add

Step 3: Add taxonomy term reference field to it.Check the settings in screen shot


once you click on save it will take you to next config page,where you will be choosing the vocab you created in step 1


Clicking field settings will take you now to last configuration page as in screen shot