I think I'm just restating what others have said, but the answer is that what to log depends on what the purpose of your logbook is to YOU. If you're a recreational diver, and not a Navy SEAL or a person whose job is to scrub boat hulls or whatever, then you're going to log dives for reasons that are important to YOU, not to an employer or someone else.
Since no matter what criteria you decide to impose on yourself you'll quickly have more than enough dives to satisfy wary divemasters who are hesitant to allow inexperienced divers to do challenging dives, how you continue to fill out the logbook for years after that depends totally on your own objectives. I, like many people, use my logbook primarily to help recall marine life and dive conditions so I can give a "review" of the site if asked. I suppose that having one's last few dive profiles written down can also be useful to medical personnel if you have a problem. In view of my log book's purpose to me, I log a dive if there is something worth writing down about it that I want to remember or that might be of use to others.
And yes, I'm looking forward as a personal point of pride to my 100th logged dive, but I don't fret over whether to log a dive or not, as at this point my log book isn't intended for anything but my personal enjoyment.