In the pre-3D days of gaming, clones were easy to spot. Pac-Man, Arkanoid, Space Invaders, and many others were commonly (and blatantly) copied by arcade devs to make a quick buck. Even later on, first-person shooters were called "Doom clones" and, more recently, open-world action games were referred to as "GTA clones."
Copy-cat claims on earlier gamers seem to have some validity, but these later examples have been forgiven as genre-starters. The sheer amount of effort and money that goes into game development these days generally forces developers to keep things original and innovative in at least some aspect, be it in visuals, storyline, or gameplay.
Still, games appear that "pay homage" to past titles. Two possible "clone" candidates have been released recently: Visceral Games' Dante's Inferno and Vigil Games' Darksiders. Now that the dust has settled on the critical front, they stand on opposite sides of the copy-cat line, showing what separates the "homages" from the "rip-offs."
Dante's Inferno plays a lot like God of War. Darksiders plays a lot like The Legend of Zelda. Dante's Inferno is very similar in tone to God of War. Darksiders offers a darker twist on Zelda. Dante's Inferno's gamplay is of slightly lesser quality than God of War's. Darksiders arguably improves on Zelda's formula, in terms of control and combat. I started writing this in an effort to make some important points about derivative games, but it seems that the only secret to being a memorable experience is to be good and different. So much for that supposed poignancy.
GTA clones lost their stigma as games like Mercenaries and The Getaway arrived promising different worlds and amped-up chaos. Similarly, Darksiders isn't a Zelda clone for the simple reason that it is fun and memorable in its own ways.