The old frozen heart icon was clear in its function (looks like a heavy defensive item... is a heavy defensive item), fits its theme (frozen piece of heavy armor) and looked powerful.
The new frozen heart does not look like a powerful end-game item, does not clearly convey its function and does not fit thematically. In fact I cannot tell what type of object it actually is due to it looking more like a mage item. 
Sharjo4 votes
...
Looks like an icy heart nestled in a nest of more fragile ice. I'd say it looks fine.
Plus Frozen Heart always sounded like something you'd wear around your neck as opposed to wear as armour anyway.
BluePolarizer2 votesThe purpose of an icon is to 1. reflect functionality 2. aesthetically fit the name 3. look cool.
The new frozen heart fails in #1 and arguably in #2 relative to the old icon.
Tesla Effect4 votesIt actually looks like a frozen heart rather than a frozen shield which was the previous icon.
Wind Whistler1 votes[{quoted}](name=Tesla Effect,realm=NA,application-id=6kFXY1kR,discussion-id=9u9fV2o1,comment-id=0002,timestamp=2016-05-10T10:17:27.307+0000)
It actually looks like a frozen heart rather than a frozen shield which was the previous icon.
How the hell do you know what a frozen heart looks like? Murderer detected.
Tesla Effect1 votes[{quoted}](name=Wind Whistler,realm=NA,application-id=6kFXY1kR,discussion-id=9u9fV2o1,comment-id=00020000,timestamp=2016-05-11T03:23:35.902+0000)
How the hell do you know what a frozen heart looks like? Murderer detected.
There are pictures of frozen hearts on the net, plus just type in "Arthas frozen heart" and you will find out.
ShinigamiKazuki3 votesIt looks like some sorta staff.
Sometimes I wonder the random direction that Riot goes in.. like.. why?
I guess it looks pretty with its colors, but I can't make out WTF it's SUPPOSED to be.
The old item picture looked pretty much clear on what it was SUPPOSED to be.
DwarvenGiant1 votesI guess it looks pretty with its colors, but I can't make out WTF it's SUPPOSED to be.
Well, I think it might be too obvious, but it seems like a literal Frozen Heart. Perhaps it's the heart of some ancient Freljordian creatures, like the Watchers?