With Trello, when I paste a URL to another Trello card into a comment, description or checklist item, Trello automatically converts the URL to appear as a human readable link. Instead of seeing "https://trello.com/c/UqLugLoB" in my comment, checklist item, or description, I instead see the title of the card, which appears as a link.
When I am entering my comment, it looks like this: http://cl.ly/image/3V3E0e1H1D3E
Once I save my comment, it looks like this: http://cl.ly/image/2T0Q2A35012y
This feature is particularly important because, by default, in Trello, cards have human readable URLs that look like this:
However, in Aha.io, the URL to a feature card looks like this:
Nobody can look at "S-835" and have any idea what you're talking about. Yet, it is common to mention other features, ideas, etc. in a comment, description, requirement, to-do item. So, this feature would be really useful, and there's a great UI pattern to borrow from at Trello.