The style and tone you use on your blog is important. Not every blog will have or should have the same style and tone. Both the tone and style of your blog is determined by your topic and your target market.
For the most part blogs should be conversational in tone. You want people to feel comfortable on your blog and not feel that they are being lectured to or "spoken" down to. The more conversational in tone your blog is the more comfortable your followers will feel.
Of course, this does not mean that every blog should be casual in nature. Do not confuse conversational with casual. Though it is perfectly acceptable for some blogs to be casual in tone, not all blogs should be.
Take for example blogs by professionals such as doctors and lawyers. Though the style of the blog should be conversational in tone, the style should not be too casual. Consumers expect a professional to remain professional without being didactic.
Slang, Expletives and Humor
The use of slang is often considered casual. Though this is appropriate for some blogs it is typically not acceptable for corporate blogs. You want to be conversational without crossing the line and becoming overly casual. Avoid the use of slang on professional blogs.
Expletives are never appropriate for corporate or professional blogs. You should avoid them at all costs. In fact you should avoid using any inflammatory language on your blog including racial slurs, slurs about gender, sexual orientation, political affiliation, disability or religion.
Humor is certainly something you can use on a blog including professional and corporate blogs. Humor can be appropriate but again do not use humor based on gender, race, sexual orientation, religion, disability, etc.
Put the Conversation in Conversational
Your blog should invite commentary from your readers. Invite them into the conversation and you will get many more loyal followers. You may have heard of the expression Web 2.0. This basically refers to user-created content and experiences. As a blogger you are engaging in a Web 2.0 environment if you invite input from your readers.
It is very important to let your valued readers know the rules for commenting on your blog. You can specifically forbid flaming (attacking other readers); abusive language; expletives, etc. It is perfectly acceptable for you to expect your readers' comments to fit the style and tone you set up for your precious blog.