The Zelda franchise is probably the one that has the most changes among the core series titles than any other Nintendo franchise...

Zelda 1 and 2 were fundamentally different from one another. Link's Awakening was similar because, well, it was the first portable Zelda and that was all that it needed to be. Link to the Past was everything Zelda 1 offered and just so much more while Ocarina of Time did that in comparison to Link to the Past as well. Majora's Mask was similar to OoT, control-wise, while being very fundamentally different in progress than any of the previous Zeldas. Wind Waker was close to Ocarina of Time, but with a completely changed overworld and visual aesthetic, with Twilight Princess being close to Ocarina of Time and by extension Wind Waker. Skyward Sword? Similar progression scheme as Twilight Princess, but completely aesthetically overhauled and with a brand new control scheme that I greatly enjoyed (and more emphasis on combat, which was nice). And, Jesus, the DS ones were even more different.

If anything, I think the series needs to find some coherence, because each of these games have had a LOT more variation (both visually and control/progression wise) than, say, the 3 New Super Mario games. My favorites were Link to the Past and Ocarina of Time, and while Twilight Princess was by no means a bad game, the lifeless (and huge) overworld and items that were used in one dungeon and then done dragged it down a fair deal. I enjoyed Skyward Sword a good deal but it still just felt like there was something missing; something the earlier games captured but the newest ones have not. I can't put my finger on it.