I've struggle with this for a long time. This is somewhat related to my notetaking struggles also. Let me offer a suggestion: that service you posted about doesn't really do anything differently than Evernote. Evernote can do all that and more, and it does it really well.
I've always thought Evernote would make an excellent journal/diary application. You just open it, write whatever you want, and close. The date will automatically be recorded, and that's the minimum requirement. you don't even have to tag it, organize it, give it a title, or anything. Just open it, write, close, done.
Now, here's the great part. Evernote can work with just about ANY device commonly used. Your computer, obviously, but both PC and Apple work. Iphones, ipad, other cell phones...they can all use evernote. Now, let's say your device can't do evernote for some reason, and here's the amazing part...all you have to do is write an email to your own specially assigned evernote address, and it will automatically get added to the database of notes.
Finally, ALL this information going back and forth from various places and devices, will be automatically synced. Just amazing. Think about it, what more do you want for a journal?
now, let's say evernote doesn't have enough specific journal features. then just export it to a more robust application. But if that's a strong need, then you're not trying to journal quite as casually as you've stated in the original post. Then you can use some hardcore journal app (like the Journal). but then, you probably lose all those amazing conveniences I described above.