I think taking what is good about a game and improving on it is fair game. I mean, I play League of Legends a lot and it is arguably a "DotA clone" this kind of thing doesn't bother me as long as I have fun playing the game I'm playing. A game, like a manga, is fun and interesting or its not. The ones that are not won't last.
There's a difference between taking some one else's work and putting your name on it (Like putting a cutscene into your youtube video. That's for you, [user]Tr1age[/user].), and using something as a starting point to make the product better. The former is theft; the latter is smart.