Google is fond of saying that all you really need to do for SEO is have great content.
Except of course that isn’t really true, as many videos and articles from Google themselves point out.
Yes, you need fresh, original, relevant content to rank well.
But there are three other categories of issues that do affect rankings to a greater or lesser degree.
Inbound links is one.
A second could be called signals – the clues that Google looks to cue it in as to what a page is about. That is the title tag, use of bolds and underlines, internal links, headlines and sub-heads, and the like.
The third is purely technical matters. There are many such. But the first rule is, if you have great content and a large enough website, you should look into possible technical issues that may be confusing Google, making your site hard to read, or giving it the wrong idea.
Of course the most extreme example of that is building a site all in Flash. You just aren’t going to rank well.
Material buried in sub-directories, many links deep from the home page, is just not going to be considered important. Google now considers blog pages to be much less important than other pages on a website, all other things being equal.
Failure to use 301 redirects when you redo a website, can cost you rankings for months while Google gets it all sorted out.
Google can be confused by finding the same page under several different URLs. This can be a problem with dynamically generated pages. Google has written quite a bit on this subject and offered up more than one remedy such a the “canonical” tag. So they must think this is important.
One specific instance of this is www versus non-www versions of URLs. Usually no big deal. But if doing a site:{domain name} gives much different results depending on whether you include the “www” then you are probably dealing with such a technical issue and better deal with it (most easily probably by rewriting the non-www version using “mod rewrite” (if you are lucky enough to be on a Unix server).
Sorry for getting a bit technical there. If you need to deal with these issues, you can find lots of detailed information online, and plenty of people who know how to handle them.
The main thing to know is this (quoting myself for emphasis):

If you have great content and a large enough website, you should look into possible technical issues that may be confusing Google, making your site hard to read, or giving it the wrong idea.