Currently "Watch this thread..." is turned ON by default when creating a new thread.
It is also turned ON by default when posting a reply to a thread. When using the traditional quick reply one does not have the option to not automagically subscribe to a thread.
Being by default subscribed to threads, especially via quick reply, generates a number of alerts. The sheer volume of alerts this will ultimately create will make alerts useless for practical purposes (since not all are easily displayed). Active users on active forums could over time be receiving dozens, hundreds, and potentially thousands of alerts. This could potentially also present a performance problem when many active users are by default subscribed to every thread they every post in.
I suggest a two step fix. Step 1 would be to change the default behavior to not watch threads one creates and not watch threads one replies to. Step 2 would be to add an option to the usercp alert section where the user has the option to change the default behavior to a) do not subscribe to threads, b) only subscribe to threads the user created by default, c) subscribe to all threads by default.