There is nothing worse than a good journal that you can't read due to an overpowering background or a too-small font size. A Cast of Characters page. This way, your readers know who you're talking about. Major players in your life should be listed with a blurb of who they are to you. A good Spell-Check program.
Typos are inevitable.
However, if you can't spell 'mouse' without a dictionary, it can be frustrating for your readers. For more information on online journals, Diane Patterson has put together some terrific tips on her site. What To Write A journal reflects the personality of its owner. As a result, no two are exactly the same with regards to content or design. Let your journal reflect who you are and what's in your head.