In my 10 years of blogging, I've made all the mistakes. They are:
lack of original design, with a lack of any design to begin with
displayed Google Ads
in my defense, not to anybody I would consider a regular or potential reader. That is, only search engine referrers. The time setting them up and maintaining them was not at all worth the effort.
covering too many topics, i.e. no focus
I did use other blogs to cover specific topics, but those fizzled with my waning interest
hello world post. Mine was short and sweet. Some are overwrought with big plans that never fulfilled. Best just to launch into it without introduction. If you don't sustain it, that's fine, the world will keep spinning.
spending so much time tinkering with code (PHP, HTML, etc.), making the site look good for robots instead of making writing the focus.
copy-edited beforehand, preferably by someone other than the writer (we miss our own mistakes)
The perfect blog looks a lot like Daring Fireball:
distinct and timeless branding
not that his blog is perfect:
when he has an update, he boogers it up with "UPDATE" in all caps
typos are fairly common. Corrected quickly (an advantage of a large readership) and his Markdown often shows, leading one to wonder if he presses the preview button in his CMS.
principled stands against Flash, meaning never embedding videos with it
little if any tinkering
design has been stable. All major changes are invisible (e.g. HTML5) or in-jokes (changing the DF logo to the Yankees)
mix between short links with lengthy-enough quotes
Gruber sometimes quotes the conclusion, leaving too little to the imagination with the effect of the reader not wanting to visit the external link
covers one topics (or a very few topics) very deeply