I was shocked to discover that the built in "Create a post" screen in a blog subsite allows for the post to be associated with a category but has no tag functionality. Is there an easy way to add tags to a post as it's created or does that have to be customized? I'd like it to allow for new tags to be typed in but also auto-complete tags that already exist in the tag cloud.
Yes and no (to the customization question). The easiest way to key tags like support is to enable on the Posts list via List Settings > Enterprise Metadata and Keywords Settings and make sure it is checked to "Add an Enterprise Keywords column to this list and enable Keyword synchronization". That shall add the generic column for tags.
For the automatically adding this you would either need to change the default Post content type (you need to enable the "Enable management of content types" at 1st) and add that column yourself, where as for the default Tags, update the column at the site level to include default values - but that shall be reflected everywhere the column is used on that site.
Other more advanced solutions would require going for code based solutions.