My first plugin, wp_favorites, constructive crit?

Hello, I am putting the finishing touches on my first plugin, called wp_favorites, and I would like your advice to make sure that I am not doing anything really wrong or dangerously (0_o). The goal of the plugin is to allow logged in users to favorite a tag or category, and then display a list of their favorited tags or categories with a template tag. This is my first, so I am sure there are some code smells and and other bad practices to iron out, do you think you could take a look? I understand not many people want to read through someone elses code if they don’t have to, but maybe you could just look for any outstanding issues.

Here is the source code:
https://codepad.org/pJ7UNney

Thanks, Joe