For detailed infos you can look at the build 13241 on MMO Champion.
Light of Dawn now uses Holy Power instead of Mana. Now heals up to 5 targets. No longer has a cooldown - Consumes all Holy Power to send a wave of healing energy before you, healing up to 5 of the most injured targets in your party or raid within a 30 yard frontal cone for 1008.96 to 1124.15 per charge of Holy Power.What does that changes do? Both a buff and a nerf.
No more cooldown is a buff, even if dependent on Holy Power, getting 3 stacks of HP is usually much faster than waiting 30s.
It can now only heal 5 targets, whatever the positioning of your raid. This is clearly intended as a 25-Man raid healing nerf, you will never get more than 5 people in your "aoe cone" in a 10-Man. Blizzard is nerfing most aoe healing recently, especially the paladins which seemed to do too much of their raid healing via aoes (that's a change ...) no surprise LoD finally got the shaft too.
All in all I think that it's an interesting change, at least for 10-Man raiders which will not suffer from the nerf.
Now ... Light of Dawn is a crappy situational heal as it is right now on live. Will it be strong enough with a 3 Holy Power stack to justify its use is to be verified once these changes are live.
Up to now, most paladins are just playing with their "disco ball" and getting silly giggles on vent for using it, it's pimp, it's flashy, it's also near from useless in 10-Man where you will rarely heal even only 3 people with it (but for Festergut perhaps, if you're a lucky healer and sticking to the melee range, usually not the case of a paladin in hard mode).
We'll see what it will be.