Everyone is looking at rips the wrong way. Sure they may all be based from the same source, but that's just a means to an end. Any project that was actually created would bear little-to-no resemblance to another. Rips may be born by acquiring another person's source code, but that shouldn't be the determining factor.
Commercial Rips. These games are nearly identical. They may have slightly more, or different content than their predecessors, some things may be improved, others degraded. But as an overall product, these games are nearly identical, and are, for the most part, just a quick attempt to suck more money out of consumers. Things that could have been patched into the original game.
Left 4 Dead vs Left 4 dead 2
Assassin's Creed 2 vs AC Brotherhood
Dynasty Warriors vs Samurai Warriors vs Devil Kings
Fallout 3 vs Fallout New Vegas
*BYOND: Zeta vs Finale
Commercial Fan Games. These are rarely rips/clones of each other. More often completely different games that just happen to feature the same characters/story themes. These games are based off of some source material, sometimes other games, sometimes non-game material like TV shows or movies. They may use the same base material, however, they almost always provide radically different game-play experiences.
Kingdom Hearts is a fan-game of Disney movies + Final Fantasy games. Its game-play, however, is entirely its own concept.
HandHeld KHs should all be considered Fan Games of KH1&2. Though these games attempt to follow the same gaming concepts set forth by the KH series, they fall pretty flat, and don't offer nearly as fluid an experience. On the other hand, they bring their own ideas, game-play mechanics, characters, etc. While still being based off of the original KH concept
Sword of Etheria could also be considered a Fan Game of Kingdom Hearts, as it employs remotely similar gameplay mechanics.
Commercial Bleach/Naruto/DBZ games are fan games of the anime, but not of other games. They each offer completely different play and graphical styles.
FF Dissidia. This is a nearly perfect example of a fan game of a game. It takes characters and some concepts from another series (Final Fantasy) but offers a completely different gameplay style and compilation of stories and characters. The same could be said for Super Smash Bros.
*BYOND: REO vs Resident Evil
Commercial Sequels. IMO, for a game to truly be a sequel, it has to improve upon almost everything (mainly graphics and game-play), reuse almost nothing (aside from the basic concept), and add completely new content/story/characters/mechanics/etc.
Assassin's Creed 1 to 2. This is a nearly perfect example of what a sequel should be. Better graphics, a new story and characters, new settings, massively improved almost every aspect of gameplay, new systems added, etc.
Kingdom Hearts 1 to 2. Almost the same sequel quality as AC2.
Final Fantasy Series. Though these games all tend to use the same basic concepts, their content, and some game-play, is almost entirely changed for each new rendition.
DBZ/Bleach/Naruto Games of the Same Series. ie: DBZ Budokai 3 compared to 1 is a pretty massive improvement to practically every system, along with adding tons of new content, gameplay mechanics, graphical style, and better overall appeal of the game.
*BYOND: Heroes United 1 vs HU2