Accomplishment. Overcoming obstacles through one's own skills.

A source of evil is a serious threat to the world. A weak, young boy goes on a dangerous journey to set things right. Over time he (i.e. the player) grows his skills and becomes stronger, to the point that he is able to defeat the evil being.

If Zelda is defined like that, then it makes sense why people feel the series is losing its charm. Enemies in general are not a threat anymore and bosses often times even less so. Sometimes it's even more of a puzzle than a real fight and puzzles, by their nature, aren't dangerous. With a lack of challenging enemies that can kick your ass, the player doesn't need to refine his skills to beat the game. Instead of "Yes! I finally made it!" it's more like "So... it's over. Okay then... time to move on to the next game."

You know, once upon a time Zelda was more like Monster Hunter in terms of difficulty or at least not a walk in the park (refering to the easier A Link to the Past here which still wasn't all that easy). What Zelda needs back is the player getting the feeling that he might not make it all the way through to the end (not refering to boredom here).

This also defines the Zelda experience:

Of course.

Not only because it serves for a greater feeling of accomplishment, but also because it gives sidequests and exploring the whole game world a real purpose. If enemies aren't pushovers, then collecting heart containers/pieces and looking for weapon upgrades aren't just a means for 100 % completion, they actually come in handy as you progress through the game.

A good example of this is the Red Ring in the original Zelda versus the Magic Armor in Twilight Princess. You can finish both games without getting those items, but you very well know that the Red Ring is significantly more valuable. Not just because it has a permanent effect, but because it is really useful due to the nature of the game throwing numerous dangerous enemies at once at the player.

Or in other words: In the older Zelda games you were exploring the land because it felt rewarding and was an integral part of the experience of a young boy overcoming a big challenge. Nowadays the exploration is done almost solely for the sake of completion.

Zelda games becoming more and more puzzle-like doesn't only hurt the dungeons, but the overworld as well. Why should you spend hours searching for stuff you are never going to need?