If you're looking to cultivate this into a long term project, I'll offer some tips:
1. Notifications are going to make or break you. I had signed up for OhLife.com 2 years back and it emails me every single day with a gentle reminder to write about my life. It also chooses a random entry that I've written in the past so that I actually look forward to reading what banal things I had written about. Prioritize this over all else.
2. Support email entries. I have visited the OhLife website about 3 times in the last 2 years. My main interaction is through email. It also lowers the barrier to actually composing an entry. Your users can create and refine drafts throughout the day and only send it across when the reminder email comes in.
3. Create a mobile app (lower priority than the above two). I've found that my most profound thoughts and exciting experiences are when I'm traveling. A mobile app is exactly what one needs during those times (Note: I use the word "app" loosely. It could just be a mobile-friendly UI for your website)
4. With apps/websites like these which solve one specific problem, it's always best to keep the feature set very minimal (even though as a developer your likely motivation will be to keep working on new features).
Good luck!