I think the thread preview should be available in the watched threads by default. "if you're watching a thread, you must have already seen the first post" is a very weak reason to turn it off. What about a newly bumped thread from 5 years ago? If you have hundreds or thousands of watched threads, you won't be able to remember what's an old thread is about just by looking at its title. Also, there could be threads with the same title created automatically by some forum feature (contact form submissions, reported posts, etc).
Also there is a consistency reason. Thread list should be consistent across all parts of the forum. But it's for another thread.