1) Thou Shalt Speak in Thine Own Voice.
(Everyone has a natural voice when they write. Some people are cranky, some are funny, some are strictly business. All voices can be ok, just make sure the voice you use is your own. If you try to sound like someone or something you're not, your readers will spot the fake right away.)
2) Thou Shalt Edit For Spelling, Grammar, and Clarity.
(Why annoy your readers with misspelled words, bad grammar, misplaced modifiers, and ... [unfinished sentences.])
3) Thou Shalt Respect Your Reader.
(Always act like the reader is smarter and more noble than you are. If you treat your audience with disdain, they will wander off, bite the hand that feeds them, or worse.)
4) Thou Shalt Not Steal.
(Pretty cool, eh? I get to use one of the ten original commandments. Oops. Wait a minute, am I stealing someone else's material? Well, even if I am, you shouldn't. Fair use lets you copy small sections of other sites for purpose of quoting, so give attribution.)
5) Thou Shalt Be Faithful.
(OK, you should be faithful. Your readers are counting on getting their daily dose of cool from you. If you don't deliver they will be disappointed. Disappoint them too often and they will stop coming.)
6) Thou Shalt Make The Choices Clear.
(The internet is hard enough to use without oddly marked and confusing links surrounding your posts. Make sure your design and copy make it easy for your readers to do the right thing. Decide that your blog will be a delightful adventure for your readers, not an infuriating puzzle.)
7) Thou Shalt Be Truthful.
(This should be easy. Don't write or repeat things you know or suspect to be false. Eager bloggers can quickly become a pitchfork wielding mob in search of a victim. Look for trustworthy authoritative sources. Take deep breaths. Think before you post.)
8) Thou Shalt Be Consistent.
(Not the same as faithful. Make sure your audience can follow your thoughts. If you spend most of your time blogging sports, and then one day decide to switch to the topic of classical music, you may confuse and disappoint your readers.)
9) Thou Shalt Not Jargon
(No Techno-speak, Marketing-speak, or Finance-speak. While every profession has shorthand language for communicating domain specific knowledge, these terms are not easily understood by those outside the field. If your audience includes non-professional readers, then write in a language that they can easily understand.)
10) Thou Shalt Have Fun
(You're probably not getting paid for this, so take your compensation in delight. If it's not fun, try something else. If you are getting paid, you'll probably find the job easier and more rewarding if you are having fun while you do it.)
Why are there only Ten Commandments of blogging? Hey, who am I to argue with Moses? Ten should be plenty.