There are two stories of videogame "plagarism" going on lately. One is the story of Zynga copying Tiny Tower and making Dream Heights. Now, a few wise asses have laughed this off by saying that Tiny Tower is a ripoff of the 1994 Maxis classic Sim Tower. But let's be clear here: There's a difference between "This game is based on the same thematic concept as a game from eighteen years ago" and "This game is a click-for-click recreation of something on the market now".
We can't expect every game to be based on some radically unique idea, but we should expect new games to bring something new to the table. I would hope that most games begin with the designer saying, "I like this idea, but I think I can do it better." If a game is simply a duplication of what is already available (and even on the same platform!) then from the player's perspective there's no reason for it to exist.
The other story is about Spryfox, the two-man team that made the Facebook / Google+ game Triple Town. (Disclosure: I love me some Triple Town. It's a relaxing puzzle game that's low on the micro-transaction annoyance factor that social networking games are notorious for.) According to this post from CEO Dave Edery, they approached the company 6wave to publish Triple Town. Instead of publishing the game, 6wave made a competing clone called Yeti Town.
Now, when these stories came out I saw a lot of posts from fans who wanted to see Zynga get sued, or wanted to see a law passed to prevent this from happening. I can sympathize with this point of view. I am 100% on the side of the people making original creations, and I hope the copycat developers die penniless by the dumpster behind Quizno's. At the same time, I'm glad we're not seeing a lot of laws and lawsuits being thrown around, because that cure could end up being worse than the disease.
(Spyfox is taking 6wave to court, but since the two companies had an existing agreement this lawsuit is more than just "I'm suing you because your game is too similar to mine". I think they showed a lot of restraint, even though they decided to go to court in the end. Read the details if you want to know the whole story.)
Let's imagine two different games, both of which use the Minecraft-style world of 1-meter cubes:
The player cares for a small village. They dig for resources and fight off werewolves, vampires, flying gargoyles, and Frankenstein monsters. They work their way up a tech tree to build steam-powered clank robots and primitive firearms. As they progress, the village grows into a town, and eventually into a bustling gothic city.
The endgame has the player build a zeppelin where they fly away with a few citizens to found a new village in a new stretch of frontier.
Compare this to:
The player digs for resources like rubblestone, iron, and diamonds. They build structures from their own imagination. Along the way they have to fight off stalkers (green armless suicide bombers) watchers (shadowy figures that only attack when you look at them) skeletons with crossbows, undead, and giant arachnids.
The endgame has the player journey to a dimension called "The Finale" where they fight a flying drake.
Let's also imagine that, due to low-res textures and muted colors, Steamtown looks superficially like Minecraft. At the same time, Digcraft uses bump-mapping, bloom effects, and particle effects to make a world of vibrant colors.
Now, those of us who have played Minecraft can see that Digcraft is an exact mechanical copy of Minecraft with a different visual style, and Steamtown is a different game entirely with a similar visual style. Steamtown is a new game, and Digcraft is an obnoxious knock-off.