Speaking as a blogger, I can point to one strength of the current system over a tag system: When you're writing a new entry, it can be really hard to remember what tags you've used in the past. So you end up with both "The life of Jesus", and "Jesus' life and ministry", and "Jesus' ministry", etc. Having the list of previously-used topics is a major strength.
If you do go to a tag system, it would be good to have some way to select from the previously-used ones:
1.) An auto-complete in the "enter the tags" text box.
2.) A handy display near the text box, that shows what tags have been used.
Along the lines of #2, maybe you could keep the "Topic" drop-down menu, but have a textbox for optional additional topics. That way, we could consult the drop-down when adding topics.