My thoughts on the matter:
It would already make me happy if I could password protect text with the web version (I'm using linux so there is no desktop version).
Protecting entire notes would be great. I would use that on a handful of notes that contain extra private stuff (passwords etc.).
For me it would be enough to just put a password on a note, without the note being really super encrypted or save from a person with technical knowledge. I would like the password to protect from a person happening to use my computer with evernote open and clicking around. So just superficial protection would be enough for me.
I'd be curious what's the difficulty in adding such a feature.