By request, I recently created a vB modification for a client that allows users to automatically subscribe to threads by its tags. It greatly enhances the user experience in my opinion and encourages posters to enter tags since it means they will most likely get a faster reply. Incidentally, it cuts down searching which helps the overall performance of the board. Overall, this is a nifty feature.
I am going to assume you do not currently subscribe to threads because of the e-mails and/or laziness.
Unlike regular subscriptions, this does not require action from you except setting the initial tags and notification type (pm/email-instant/email-daily/email-weekly). A good example is the mod I recently posted.
Edit: A variable number queries are added. This is proportional to the number of tags for the new thread being posted.