As someone who works with intellectual properties, and someone who knows a lot of people who also work with intellectual properties, I can safely say that a lot of us like to keep our ideas to ourselves. There is a fear that if your idea is leaked, then there’s a team of coordinated and efficient coding enthusiasts that will take your idea and do it faster than you. Probably better than you too. And they all wear sunglasses and matching black leather uniforms with lightning bolts...
Where was I? Oh, right, the fear of being copied. This fear has a less-paranoid-but-still-paralyzing twin: the fear of someone coincidentally doing the same thing as you, making you look like the copycat nonetheless.
My advice to you is to fight that fear and show off (or at least talk about) your works in progress. There are several reasons why this will help you rather than hurt you.
1. It Builds A Buzz
People like seeing the behind-the-scenes view. If people can see your project coming to life, they’ll be more engaged with you as a creator. They will also be anticipating the project's completion and release, so when it's ready, you’ll have an eager audience.
2. It’s Evidence
If it so happens that someone else has the same idea as you, you will at least have proof that you’re not copying them. Alternately, if it turns out that someone actually IS copying you, your ongoing display of announcements and screencaps is your proof that you had it first.
3. It’s Advertising
You know how games and movies and even sometimes books come out in waves that follow the same style or theme? These properties will always be compared to their contemporaries. So what if someone else happens to come out with a game similar to yours, almost at the same time? That happens even to the biggest players in the industry. Whether Game X or Game Y is more popular depends on which of the two is more prevalent in people’s minds. Therefore, the more you tell people about your project (even before it’s ready), the more people will think about it. At worst, it will give you a little nudge above your competition. At best, it will make you the “Street Fighter” while the others become known as “Street Fighter clones.”
4. It’s Market Research
If you keep a blog of your work in progress, you may get commenters. Okay, it will probably only be a comment or two from your friends, but it’s comments nonetheless. They add up. Occasionally, the nature of these comments will be “that works,” or “that doesn’t work.” Big established companies dedicate time and money to building focus groups. You’re getting one for free, and you’re not even done your Beta yet.
I hope this encourages you to post bits of your work in progress.