In the beginning, spec scripts were written by new writers to demonstrate storytelling skills which could lead to writing assignments. Many writers with a script and a dream look to the spec market as their only hope. Since the market is always changing, it is hard to keep up with what the buyers are looking to buy. In an attempt to appeal to the tastes of potential buyers, new writers will often make the mistake of trying to predict the market. They tend to pay much more attention to the concept or action driven material, but I think everyone has a chance.
Some genres fare better in the spec marketplace than others.
Action seems to do well, as do thrillers and other high concept scripts. If I had to advise anyone as to what to write I would write an action-adventure piece. With those odds, a writer who can write a 'good actioner' has a better chance. The operative word is 'good' since just about anyone can copy the formula of Die Hard and write a script. Good writing may not guarantee a sale, but it will help a writer get noticed.