I use to like video games when they have a more adult and serious tone. Cheesy cutscenes and upbeat elements just turn me off. Sure, I love good endings and stuff, but I don't really fit into a world where world desctruction is depicted by laughable main characters and an aloof villain such as what (to give an example) Eggman is nowadays. He was so awesome in the Adventure games!
I think Zelda gets away with what Mario, Kirby, and many other Nintendo franchises do not achieve: seriousness (in their main games at least) . And that is why Majora's Mask and Twilight Princess are my favorite games in the series. First of all they are REALISTIC (graphically speaking of course.). And that's very important for me. If you know me you should know by now cell-shaded graphics bore me to death. Apart from visuals, Twilight Princess was epic, and had this sense of despair that just caught me instantly, and Majora's Mask had this surreal elements that left me for days thinking of what was the real meaning behind some areas and cutscenes (the inside of the moon, for instance.) That's a mature element there, something that happens in the game that leaves you scratching your head and wondering what has just happened and what's the meaning behind all this.
I'd certainly love some adult themes on Zelda. Probably adult in the sense of mature elements in the storyline.. more character development, essentially (no gore, no sex, that's for other companies to develop). I'd also like to see some mind screwing. Examples of this in Zelda are (for me at least) the scene where Zant cracks his neck and the inside of the moon, and various other Majora's Mask scenes that leave you thinking about what is the meaning of all that. I would like to see some more of that surrealism.