It needs to be informative and to a certain extent... witty. I reckon anyway.
You have to remember a few things though...

deviantART is essentially an artist community, so people wont care what you had for dinner or where you are going for your vacation. I will.

Give the journal entry some substance, don't just say "Joe Bloggs has a new album out and it's crap!", people will be more interested to know why you thought the album was crap.

I guess really, just take into consideration what other people may see in your journal entry. Most people on the internet are stangers to each other, so they wont hold any immediate compassion or relation to you. Tell them what you think they'd appreciate hearing, not just want you want to get off your chest.

Just my thoughts, anyway.