Can one feature module make additions to another feature module? I would like to have two feature modules for users to enable. One basic and one advanced. For example, the basic feature creates a blog content type, with only title and body fields. The advanced module, once enabled adds additional fields to that original content type, such as tagging or imagefield. Is this possible or will one override the other? Or will my site blow up? or?
It's definitely possible, I had to do something similar yesterday and I'm happy to say features can handle it perfectly.

The trick is that your 'advanced' feature will not contain the content type at all, but simply a list of the extra fields that are attached to the content type defined by the original rule. 

When you install/enable the second feature module, the extra fields will be added to the existing content type as if by magic. I was pleasantly surprised at how well this works.

There is an example of a well established Drupal distribution using this same method (it's where I got the idea if I'm being honest); OpenPublish contains the openpublish_article feature, which defines a content type, and the openpublish_atricle_extra_media feature, whose sole job is to add a couple of extra fields to the article.

Just as a side note, it would probably be a good idea to make your main feature module a dependancy of the second, so the content type is definitely defined by the time the second feature is trying to add the extra fields to it.