Photos (OS X Yosemite / El Capitan) – show original in Finder? I would have expected to be able to right-click (secondary click) on a thumbnail of a picture and have the contextual menu give me the option to show the original picture document in Finder... but it's not there and I can't find it anywhere.

I couldn't stand iPhoto, but at least it would allow you to access the original file directly in Finder. 
I understand why the Apple want to restrict direct access to the originals (stops users from moving files and screwing up their library), but what if I want to do something other than email or post the picture to social media?

EDIT: I know I can just "Show Package Contents" or use the terminal to access ~/Pictures/Photos Library and hunt through the folder structure manually, but it's painfully inefficient considering all the sub folders

It may be a (deliberately) missing feature, but if anyone knows any way of jumping directly to the original instead of copying or exporting it, that would be great to know.

UPDATE: I have got used to the way Photos works by now. Basically the thing to do is just to relax, let it manage your collection and only export what you need to as a copy, which you can later add back into Photos when you are finished with it. It is certainly a deliberate design choice by Apple, not a mistake.
Photos expects you to export unmodified originals as copies, to preserve the integrity of the internal file management and for collection protection in the same way a database works – it's central to keeping the iCloud storage of your photo collection synced, stable, and functional.

Once you have exported a photo, you can then use mdls and mdfind to see what the "copy" looks like and where the original was stored.

In my case, they are all in ~/Pictures/Photos Library.photoslibrary/Masters/2015/04/08 and such stored by year, month and date for my US locale computer. You'll only have to do this exercise a few times to nail down the internal storage for each time Apple chooses to change it going forward.