Short answer: it will be back (and improved) very soon.
If you look at the refactor card menus ticket you can get a sense of what we're going for, though the exact design details are still in flux. Some key points:
There used to be a "related" view of the cards. That's basically being supplanted by the menu itself, but all the cards that used to be under that tab are still available (wonkily, I admit) as plus cards. Eg, for this support ticket you can go to:
actually happy to hear that the related tab was of some use to you. Many just found it confusing. So while I do expect to retain all its functionality, much of it will be tucked under "advanced", fyi. The good news is all of it will be one click away (whereas most of it was previously available only with a second submenu click)
--Ethan McCutchen.....2013-01-05 18:02:13 +0000
Some use!? Tagging, discussions, watchers, interrelationship of cards, etc? HUGE.
--Brandon WilliamsCraig.....2013-01-06 00:47:55 +0000
ok, I hear you. I will try to make progress on this this week or next.
--Ethan McCutchen.....2013-01-07 16:53:47 +0000
I think this is all handled now except for access to tagging and watcher lists. See some improvements to menu, especially the discussion.
--John Abbe.....2013-03-17 17:29:46 +0000