What makes a game a rehash of its predecessor? Is it really so bad for games to be rehashes?
I'll start by saying I've never understood why rehashes are so bad. That goes for franchises I like and dislike. That said, I'm curious why some games are dismissed as "rehashes" while others are forgiven for doing exactly the same thing.
As an example, I'll use two series I personally enjoy: the main Mario series of games and Zelda. With each successive release, I hear a good number of cries of "more of the same," perhaps with a note that "x gimmick" is tacked on. For example, Super Mario Galaxy is just Super Mario 64 with a gravity gimmick. Skyward Sword is just Ocarina of Time or any game released since then with a new vehicle and controller gimmick.
These series have undergone some huge changes. Super Mario Galaxy is clearly very different than Super Mario 64 and its successor Super Mario Sunshine, with platforming brought to the fore and adventure gaming reduced to almost nothing. The level design operates on different principles, gravity or not, and ensures a fair amount of gameplay variety. In the meantime, a number of Zelda games released since Ocarina of Time--such as Majora's Mask, The Wind Waker, and even Skyward Sword--eschew some elements of the basic structure while keeping others, changing some fundamental gameplay ideas, and implementing old ideas in new ways.
Having played Halo:CE and Halo 2, I wonder why I never saw this charge leveled against the second game. The basic mechanics all felt the same. I think level design was more skillful in the sequel, dual wielding made you feel a little more powerful in a visceral way, controls were tightened on vehicles, and the Arbiter offered something of a superficial change, but otherwise, it met all the criteria for a rehash. Nothing I've seen from any of the sequels has led me to believe they differentiated themselves very much, but I admit I may be missing something because I haven't played them.
The Elder Scrolls is a similar story. Since Arena, some very basic ideas have remained the same. I think Morrowind was significantly different enough that it almost felt like a different genre at times, with adventure and exploration taking precedence over Daggerfall due in part to the 3D gameplay, but since then, most of what we've really seen has been fine-tuning and reduction. Oblivion actually offered far fewer options than Morrowind, with the only major superficial change I can think of being the combat. That's entirely irrelevant to the core RPG gameplay, and wasn't fast-paced or interesting enough for The Elder Scrolls to become a more viable action franchise. And while I certainly enjoy Skyrim more than Oblivion (though not more than Morrowind), it continues the tradition of fine-tuning. The leveling mechanic is changed entirely, but how much does this affect how you play? I guess it depends on the type of gamer you are. At a distance, these games seem to operate on the very same ideas. I don't see how they're any less rehashes than Mario or Zelda.
So how is a rehash measured, and why is it a bad thing? If any of the four franchises mentioned above are built on rehashes, I don't see why anyone should complain. After all, they're all respectable, and all have solid games.