This is sort of silly... Are people in any way surprised that two games with "Zelda" in the name have the same core gameplay? Two games with "Tetris", "Metroid", "God of War" or anything else in the title? Of course the core gameplay is going to be the same; that's why you bought the first one - you liked the core gameplay. If that changed, you would have an entirely different game that would upset fans of the series regardless of whether or not the game was good, simply because the game was not what they're looking for when they buy (say) a Zelda game.
The trick (and the best sequels are those that can successfully pull this off) is to keep the essentials of the core gameplay intact, and change other things up enough so that it doesn't feel like any other game before it. Sure, you save a princess in almost every Mario game ever, but the abilities at your disposal, the obstacles and terrain you must negotiate, the characters you encounter, the art style, music, gameplay concepts (water pack for Sunshine and the effects of space, spherical land-masses, etc for Galaxy) and numerous other aspects of the games often differ between titles. In this way, the game is never the same, but always has the element that drew you to the series.
The only series that deserve to be pointed at angrily are those that do not evolve those parts of the game that are non-core, those developers that don't take the time to think of new ideas to make the next game different and therefore worth playing.
"Whenever you find a man who says he doesn't believe in a real Right and Wrong, you will find the same man going back on this a moment later." -C.S. Lewis
"We all make choices... but in the end, our choices... make us." -Andrew Ryan, Bioshock
Prediction: Wii passes 360 in US between July - September 2008. (Wii supply will be the issue to watch, and barring any freak incidents between now and then as well.) - 6/5/08; Wow, came true even earlier. Wii is a monster.
