Me too! I don't like it when programs insist on storing my photos in their own, non-human-readable directory structures and file names. Otherwise it becomes a nightmare trying to use them in other programs. Even iPhoto->Photos managed to break my library.
I just wish that Google would bring back the simple online photo sharing that they used to have with Picasa. The Google+ merge ruined all that.
If you're willing to keep your photos in Dropbox and have the space for it, take a look at Carousel.
Carousel respects whatever folder naming convention you have. All photos in your dropbox show up in Carousel's photo list.
Pictures on Carousel are always saved to the "Carousel" folder in your dropbox, but you can move them around to respect your preferred folder structure without damaging anything.
Not sure. My intuition is that photos are completely read-only; I don't think it will ever change them, so tags might be stored in some other opaque storage.
I do know that Carousel uses the DateTimeOriginal EXIF tag to set the date. You can use this command to re-tag photos, for example:
I just wish that Google would bring back the simple online photo sharing that they used to have with Picasa. The Google+ merge ruined all that.