Q:

Follow Users and Tags like for Questions

It is possible to follow questions via RSS feed at the bottom of a qestion page.

This would be helpful for users and tags as well.

A:

It's already in the system. Most RSS readers can find the subscription URL by pasting the tag page and user URLs into your reader. Or, you can create them manually.
For tags, the URL
https://stackoverflow.com/feeds/tag?tagnames={tag name}
allows you to watch a tag. Just add your tag name and you're off.
For users, the URL
https://stackoverflow.com/feeds/user/{user ID}
will let you follow a user's comments, answers and questions.  

