Good content - the user and their situation be the judge of that!
Who are you writing for? Why? Are you writing because you want something or because you're giving something? What are you giving? Who wants it? How many of them are there? Where are they? How will you get to them?
Content that would be worthy of people liking or sharing would be what I'd define as good content. If you have a site about dogs, and you are writing about wedding tips, you may be stretching to find a link between the two. Your content should be somewhat relevant to your typical customer. i.e. if you supply part to mechanics, then posting recipes on your site is probably pointless. (these are extreme examples, but help to prove a point)
Blogging is not just to write, not to impress rather the ability to clearly communicate, engage and write compelling content. This is how I justify blogging and its importance online. I believe that good bloggers have a unique variety to attract readers over time. Mostly, it's direct to the point, keeping it short, fresh ideas and inspire others.
If your blog is about business related topics such as SEO, email marketing or site management, then you should stick to that niche. If you have a general (news) blog that covers a wide range of subject, then you have more range to post about topics not related to one very defined niche.