There's always a mole. It's one of the things you can bet on each season with 24.
There's always a mole.
Jack will always get injured and then be fine 2 hours later.
No one ever sleeps.
Someone close to Jack always gets into danger with the terrorists.
CTU usually has some kind of damage done to it.
The list could go on and on. I wouldn't be surprised if the writers had a list of criteria that each season had to meet. Doesn't mean I don't love the show any less. Sure they retread story elements, but the show's a pretty good ride each season and that's why I love it and watch it.