The relatively new forum feature "tag" is interfering with the old feature "thread subscription".
The interference is of a graphical nature. The new tag icon looks very similar to the subscription icon, and it's confusing. No one cares about thread tags anyway, and i suspect the have very little search value too. The subscription icon on the other hand lets use easily spot the threads that you replied to, so you can check these threads again, and it's actually a pretty useful feature.
Example:
