Predefined list of tags and genres
When searching and adding tags and genres, you should allow selecting from a list of predefined values. This has multiple benefits: avoids misspellings, standardizes data, helps the user choose. It also lets people know that you *can* search by tag and genre; this is not at all obvious in the current interface.
You could either populate the list yourself, or choose from freeform entries that users have already chosen.
I would suggest having a fixed set of categories - basically genres that you put each comic into. This will really help if I'm trying to explore say "comedy". Separate from that it would be nice to let the users tag the comics freeform. So Sinfest gets "comedy" as a category AND a bunch of more specific freeform tags from the users.
Some examples of other sites categories would be Clicker.com (what you're doing but for video) at http://www.clicker.com/category/action-and-adventure/
or Good Reads (for books) at http://www.goodreads.com/genres
AdminJamey Sharp (Admin, Comic Rocket) commented
You're definitely right, and I've added this to my to-do list. My current plan is to add an autocomplete dropdown but still allow free-form text. Does that seem like the right plan to you?