Write what's in your heart. If the story is good, it'll be bought.
Sadly that's not true; if it was right now I'd be halfway through shooting my second feature and I'd still own my own home. Distributors are looking for easy returns and they won't step outside of product types that they know will be easy to place.
I think there is more room to move if you are writing a script with the intention of selling it onto a production company who can afford to attach names, but for anyone thinking of self financing the story you pick needs to have a strong hook.
My current feeling is that anyone who is making a film these days can't afford not to do market research before they start writing the script.
These are the things I would suggest:
1) Get your ideas and then check to ensure that they can be explained easily in one sentence and that one sentence is catchy enough to make you believe that you'd hand over $10 to see it in a cinema.
2) Design a poster/DVD artwork for your film and see whether the idea is strong enough to communicate itself as a DVD cover. (Then send that cover out to fifteen people you trust and ask them for comments - pick people who will be honest with you)
3) Check to see whether there are currently DVD's like your concept for hire or sale in your local stores (note down all the distribution companies that have already sold a film like yours)
4) Spend some time on the net researching similar films, what they were shot on, who bought them and what other kinds of films these companies buy. It's also possible to get box office figures.
5) Get honest about the films that were like yours (If they all had people like Bruce Willis, Nick Cage or other names, your idea is no good to you as a production vehicle - you have to have somehting that will sell without names)
This is the basic deal - if you want to know what you can sell, go to the retail outlets and see what's on the shelves.
Not only that, if your idea is so original that you can't find anything like it on the shelves then you are going to struggle to find distribution.
Genres are important to the business. Distributors are looking for an easy buck, make it easy for them and they'll buy.
Whether format is important, well I don't know; whether horror is played out (I doubt it). The problem with those kinds of question is you'll find contradictory information, if any of us had the perfect sales formula sorted out we'd be sitting by the pool taking calls from the big guns right now. Some people will tell you that only stuff shot on film sells, some will tell you that HD is the new format and plenty of people can provide you with a massive list of sucess films shot on mini-dv.
The only thing I know for sure is that making a good picture doesn't guarantee distribution, neither does spending a lot of money on it, neither does the quality of either the script or the acting. In this business the idea is the key, get a strong enough idea and all the rest is possible, get that wrong and nothing can save your project.