Good journals do have peer review, and they wouldn't have accepted this.
Also, one big disadvantage of blogs -- depending on where you host it, they often don't last long, and point 4 (easy to edit) is a blessing and a curse, particularly if people edit things without telling you they did it.
Of course, blogs have their place, I keep one myself, it's great for breaking news, explaining things in greater depth, sharing and understanding. And journals could learn from blogs / arxiv (make it easier to make corrections, allow discussions).