Content is king, it is everything! Though it still has other criterion for SEO, without content, it's nothing. What I am going to emphasize is that you must concentrate more on your content. Of course, url and heading are helpful when it comes to SEO. But, don't forget, links, urls of your site are just occupying a small little part of your site. In majority, contents take up more than half of a site/blog.

Contents

There are few types of contents. Text-written contents will face no problem when you working on SEO, however, images, javascripts, and frames, will be more troublesome. If your contents are mostly text-written, then you will not worry about search engines unable to crawl/read your content, instead, worry more of your competitors. Images, javascript, videos, clips...which are about the animations, search engines are hardly crawl on them. They don't read images, but they read text. Though search engines are getting sophisticated, guess what, they are not that advanced until have the ability to watch animation as though a human and classify them accordingly. (See also on how to add alt-text for images, for SEO [Blogspot] )

Well, search engines made a big deed——classifying billions of sites accordingly to the certain keywords. Sarcastically, why do some of uninformative sites will be placed on top of the better site? It sometimes related to contents. Of course, a site which provides great informative articles is considered as a great site, but eventually, they fail to optimize keywords, and keywords-stuffed contents will seem to be gross and odd. Why? There are many reasons——backlinks, contents... But, for now, I'm going to say about contents first.

The bad and uninformative site can win over a great informative site, as they got their keywords optimized. A great site should concern of this big problem. Instead of just posting a great articles, try to place some keywords into the contents, until the optimum keyword density.

Relevancy of your contents

Relevant contents are important as long as quality is concerned. If you have a dog-adopt site, and you provide an information about cats instead of writing dog-adopt related content. In a common sense, you are getting out of your theme. So, try not to get out of your theme. i.e. If you write about dog-caring, it's alright with the theme of dog-adopt which is still has some relationship. So, avoid yourself writing something out of the topic!