Deep changes in classic gameplay or lack of variety?

It's dangerous to lump the original Legend of Zelda in with the rest of the series. Although it established a franchise, the very first game actually had very little to do with the "Zelda formula" that is still in use today. Most of the conventions that have really become synonymous with the series were introduced in Zelda: A Link to the Past on the SNES. The original Legend of Zelda actually had a lot more in common with open-world sandbox titles. In many ways it was the first example of open-world gameplay.

A Link to the Past began streamlining the series and making its level progression more linear. It also first introduced the early-game hand-holding that persists to this day. It is also notable for introducing the idea that boss battles serve as tutorials for how dungeon-acquired weapons ought to be used. In the original Zelda, boss's could usually be beaten by any weapon.