Hero Kid, can you give me the OS and browser version you're using?
I just attempted to subscribe to a thread with Internet Explorer 8.0.7600.16385 and Win7 (64bit), and was able to subscribe fine. I was able to do so with Mozilla Firefox 3.6.8 on the same machine, and no issues.
According to the tech team, your default for new subscriptions was set for "don't subscribe", and then under your subscription options, you had the "Don't ask me" box checked on the "Would you like to be asked to set the frequency on each new subscription?" setting. That box states that when checked, that the system will use your default method without giving you a dialogue to pick from, and since your default was set to "don't subscribe", it wasn't.
They are looking into a fix for that so it won't be an issue, but have changed your setting so you can now subscribe.