I think there's nothing wrong with similarities between games as long as it isn't intentional or too similar. A space marine fighting aliens is one thing, but a space marine fighting aliens with gas masks and sqeaky voices might be taking a tad too much from Halo.
Then again, there's practically no such thing as an original game anymore. The industry has put out so many games that there really isn't any way to make a game that doesn't share elements with another. Similarities are unavoidable, especially when the game is played by someone who has been playing for a long time. The best thing you can do is stop loking at something as a rip off of another game and judge the game by it's merits.
That, or troll about it on a forum.