I see two topics here.
Blogs:
We never use the blog. We always use a Webcontent structure called "Blog".
While companies often want to use the term "blog" for marketing reasons, it rarely is actually a blog. Often the articles are not even written by the "authors" and rarely do they enter the articles themselves. Even if an article is written by the given author, it is mailed around, reviewed and only in the end one of the website editors . In the end, often a blog is a "general article" or a special kind of news.
Also, Structures/Template give us more flexibility when rendering the thing. The ability to add metadata e.g. specify the "actual author" is quite useful. And it also helps us in the overview ADTs since we always just need to render webcontent, we don't have to handle blogs and webcontent.
Caveat: There is an existing thing called blogs that confuses editors and we need to hide it or tell them: Don't use that.
Default Content :
I don't think structure default values will cut it. Creating a: "Blank Entry", "Newsletter", "Serious Post", "Inspirational Post" or a Olaf mentioned, a "Radio Liferay" episode, will always create a new "BlogEntry". To use default values in the strucutre, we would need to create multiple structures for that and doing that will be quite a headache from a maintenance point of view. In truth, the editor just wants to add a blog and have some stuff already filled out.
I think that this could be handled by the new applications feature. Instead of having the default editor, I would create a new application "Create Newsletter". The form would be based on the structure, with the ability to move fields around, prefill them, make them required (even if they are not required in the structure), the form wizard could help me categorize/tag it, select related content, ...
Well, I guess, the above process is where you want to go anyway, so I would rather see it there. (Even if I thoroughly dislike the current forms implementation)
