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You've been able to heart Posts and Collections for a little while now, but it was just kind of a gesture as not much was being done with that heart. Our bad - but that's been our development style - don't hold up releasing a feature just because you could make it better later.
Now hearting is actually more useful, because you can see all the Posts and Collections you've hearted in the past. It's a form of bookmarking, just like hearting Pens can be.
There was a bit of a re-jiggering of the "Loved" area on profiles, allowing you to change between any type of content and how many hearts you gave each. Here it is show all Posts I've hearted:
You can view Pens or Collections as well, see the statistics on them, paginate through them, and narrow down by level of hearts.
Exposing those statistics is new for Posts as well. You can see them on any Post, including your own: