It seems that everybody is an SEO expert, even though, if you truly understand the field, you quickly come to realize that nobody knows everything about SEO. In fact, many people who say they do, are really just guessing.
There are barriers to understanding, specific to Search Engine Optimization strategies, which make SEO a very inexact science:

1.Search Engine Companies do not give accurate information about what they penalize for, or rate high for. They choose to keep this information obscure so that results cannot be manipulated.

2.Many "experts" use anecdotal evidence because it is the only kind they can use. This means that their results are very subjective, and not accurate.

3.Statistical data is nearly impossible to obtain. Since site content varies and no two sites are alike, and since there are so many other variables (marketing, traffic happenstance, optimization variances,
etc), you cannot exactly pinpoint which strategy yielded what result.

4.Time delays complicate the picture. If you make a change in your site, results may be almost immediate, or they may take up to 6 months to show up. If you make regular changes to your site, it may be VERY difficult to actually pinpoint which strategies helped, and which ones harmed, or which ones simply made no difference, since typically we make changes more often than once every 6 months.

5.Most results are obtained from sites where the goal is to be successful, not to do comparative analysis. This means that no meaningful comparisons CAN be done, because you'll not risk harming the site by testing out a controversial tactic.

6.Many SEO "experts" issue information based largely on theory, instead of actual experience – some tactics require situations which are difficult to set up to be able to analyze with any degree of accuracy.

7.Search engines are computers. Computers are notoriously bad at using language. They can only compare word usage literally, not define the meaning. This means that judging relevancy is something that is not nearly within the grasp of any search engine company. In spite of this reality, Search Engine companies continue to promote the myth that they do consider it, and SEO writers continue to repeat it. In any given situation, a computer will judge links to be irrelevant when they are relevant, or relevant when they are not, about 50% of the time.

8.Search engine companies always want site owners to think that their algorithms can do more than they actually can. They want people to be off guard, and to feel that manipulation will get caught. This means they will perpetuate myths themselves, in order to create the illusion that they can track all forms of manipulation.

9.The goal of a search engine is to accurately index the value of sites and pages on the web. Many SEO writers forget this when they begin to speculate, and they assume that the goal of the search engine
companies is to trick people into getting banned, so they formulate all sorts of impractical strategies. This is rather silly, since an honest site owner is less and less likely to get trapped as technology
develops, as long as they conform their goals to that of the search engines – to insure that their site is ACCURATELY described.

10.SEO speculations and misinterpretations of policy are rampant. Many so called "experts" have devised all sorts of contradictory advice, which is highly illogical when thought out. Issues concerning things such as duplicate content, back links, outbound links, and other basic topics are often misinterpreted because someone has abandoned common sense. If a rule cannot be extended to all realms of the web – from small sites to mega corporations, and still be sustainable, then it is not something a search engine company would do. They are not banning all duplicate articles (or all article directories would be banned), they are not taking away some of your pagerank for each outbound link
on your page (or all directories would have a pagerank of 0), and they are not penalizing all Content Management Systems (or every large CMS managed site would be banned – a growing website group online).

11.Things change rapidly. So something that was true two years ago may not be true today. Acceptable tactics three years ago are considered Black Hat today. Sites that would not index accurately a year ago do so in most search engines today. Information online sticks around long after it has become obsolete, so while the rules may have changed, the article about it that was written years ago has not changed and is still available and being read.

SEO is not about money and fussy little technicalities, and it should not be. It is more about honesty and intelligent communication. Search Engines are not holding contests that benefit only the wealthy. Their guidelines always have been established to allow a nobody with a good idea that is well presented, to excel as easily as a corporation with lots of money behind it. I hope that never changes, because if it does, something unique about the web will be lost.

Search Engine Optimization is a highly complex, and highly speculative arena. Nobody knows for certain exactly where the fine lines are drawn.

Tactics which are manipulative, or which have been abused by spammers, are know to be a harmful influence on your site. Tactics which increase the ability of the search engine to know what
your page is about, and which help to more accurately describe what the page is about, are known to help.

Beyond that, we get into the gray waters of theory, and no one, no matter how well thought of, really knows the answer – or if they do, they have motives for not saying so.

This is not meant to imply that an experienced professional cannot help your site to rank higher. They can. But only if you hire someone who understands that the goal is to focus on the known things that
really matter, and which give you the most bang for the buck, at the level of service that you can afford. If their recommendations sound illogical, get another opinion.

Common sense, and honesty, are, as always, your two greatest guides to success with a website, in SEO, and in everything else.