I see where you are coming from, you want a cooler lvl 51 ability, and also something that scales with RP. I personally think HB is pretty useful (with glyph atleast). I'm currently leveling a DK and tanking has become very fun and easy with HB, Glyph of HB, and Killing Machine procs. Almost all my HB are crits, and my aoe tank rotation is very simple.....HB, BB, BB with frost strike thrown in randomly.
Some 51 pt talents are just meant to fill in a niche that the class was missing, like riptide for shaman was a instant hot that we never had before.....HB is dk's nice burst aoe.
My only complaints are that it needs the glyph and crits to feel that great, and also Rime doesn't seem to proc enough, but I don't oblit much as a tank so.
They should just combine the glyph with the ability, and add to the tooltip that it will increase in damage if you have more RP pooled up. So with like 100rp it will do 10% more damage.