I see my fair share of playtest and "alpha" rules and one thing that makes me smile is the paranoia some people display about their ideas. They make a few assumptions such as:
-Other people actually care about their cool idea
-Their idea is actually original
-Their idea matters even if it's not actually part of a functional, successful game
-Game mechanics are somehow "patentable"
-Their idea or half finished game is worth stealing
Guess what? Originality is no guarantor of success. In fact, as I've commented repeatedly elsewhere, most successful rules seem to be regurgitated or copy+pasted from somewhere else. Flames of War, Bolt Action, Warhammer 40K, and Warhammer Fantasy share similar mechanics. Armada is just a more complicated version of X-Wing, which shares a lot with Wings of War. Lord of the Rings: SBG has a host of imitators. Stargrunt/Dirtside have spawned a host of indie hard sci fi rulesets. In fact, originality is overrated when it comes to making $$$ out of a game. Your original idea is unlikely to make you rich. And if game mechanics were patentable there would be a LOT of lawsuits put out by GW, which is a notoriously litigious company. (Not that they didn't already steal every mechanic and piece of background fluff from other people, anyway.)
You're not original, anyway. Odds are, your idea has already been used before. "There is nothing new under the sun" declared King Solomon. I never knew the guy played wargames until I read this. You idea or mechanic is not a game. Until it's a fully working game, with people actively playing it, it's still an idea or concept. And good ideas are a dime a dozen. Heck, we ALL have had a Great Wargame Idea. A good idea is not necessarily a good game. I see lots of games with one or two awesome features/ideas/mechanics, but the game itself is lame. A good game is a fun game - one people want to play. And a finished game that is fun = a lot of hard work.
Pretty much no idea is totally original, but builds a little on what comes before. That's how technology progresses. Others having the same ideas as you is not surprising. If a majority of fantasy/sci fi gamers came into the hobby playing 40K/WFB, I'd expect to see 40K-esque influences in their games. I can see "Stargrunt" in the ancestry of about half the indie hard sci-fi rulesets made for 15mm. Just because many of the games are similar doesn't mean they are actively stealing ideas from each other or even are aware of each others' work.
Odds are you are not an unidentified game design genius. And if you only share your ideas with trusted friends/family (you know, the sort of folk who are really analytically critical about your work and will go all out to try to "break" your game mechanics <sarcasm disabled>) you probably aren't getting the depth of feedback you need. There's a LOT of work involved in making and publishing a working, properly playtested game. (Which is why I test and critique games rather than designing them myself!)
The wider range of people that playtest a game, the better. For example, I get the feeling that GW secretively playtests with small group of testers (hermetically sealed within an ivory tower?), compared to Privateer Press who did an open "beta" for their Mk.II rules. "We did a very extensive test of the Witch Hunter Codex - 8 people, 30 games - it's the most exhaustive yet."
To be blunt, your idea seldom sounds as awesome to others as it does to you. I mean, your close family/friends/gaming buddies probably say its awesome, but are they really the best barometer?
I bet I could easily make a game that is popular with the regular posters who frequent this blog. Doesn't mean it would be a best seller though.
Risk vs Reward
You know, I can't really think of any examples of copywright/IP theft in tabletop games? Or even boardgames/cardgames? I think there was one involving Magic - but that is a complete, finished, fully functional, wildly successful, published game. You know, something proven to be worth stealing. Not a "cool idea." Not a prototype. Not even an alpha/beta playtest copy.
You know, it's actually quite hard to spread word about something. You know another term for spreading the word? It's called marketing, and people actually *gasp* pay others to explain the benefits of their ideas. However many designers miss out on this free marketing in order to "protect" their ideas.
Ironically, a few RPG companies deliberately torrent their fully finished rules on pirate sites in order to raise interest and attract legitimate sales.
Also, tabletop gaming isn't exactly a big money business. The real money is made by big hitters like GW or PP, who make their money through miniature sales. Which is not exactly plagued by piracy either. Despite our passion for tabletop gaming, it's not exactly a prime target for hardcore IP thieves. They're targeting better places - industries that actually have the potential to make real money.
I think game designers are missing out on valuable chances to thoroughly playtest/improve their game, and indeed market their game, in fear of having their "original" ideas stolen.