It's not "bad" but it's kinda.. I feel it's ridiculously restricting.
Look at it this way: Your Hammer attacks now apply a Chill effect! But only when you crit... and even then half the time. And only once every 45 seconds. And it only lasts 4 seconds.
Meanwhile, my Elementalist can pop into Water Attunement, and pop out Frozen Burst every 15 seconds, for a guaranteed 3 seconds of Chill, not counting how much Chill I could apply if I used the Glyph of Elemental Attunement in Water, over the duration of the Glyph (sure, 45 second CD, random chance to happen, but it adds up to way more than 4 seconds every time).
The "When you crit, apply chill" idea is great synergy with the rest of the trait line, to be sure, but chill is still an odd choice.
All in all, I'd like to see Guardians have more effective methods of applying Chill, if they're going to apply it at all. I'd honestly prefer for it to have a random chance to do a knockdown, instead of Chill.