Mmmm infused beef. Delicious, eh?
I'm going to rant about Infusion of Light, but first I'm going to ramble about Serendipity.
Different talents are different. Different trees are different. Different classes are different. But... I didn't realize how much I disliked IoL until I played my priest enough to fall in love with Serendipity.
What I love about Serendipity
Procs by casting Binding Heal or Flash Heal. Combined with Surge of Light (great talent synergy!), I generally find opportunity to use Serendipity procs at least a few times a fight. And I have choices: I can either use it on a Greater Heal or a Prayer of Healing. And if that wasn't choice enough, I can either spend 1 stack or wait and cast at 2 stacks!
But it gets better. Priest healing spells are so diverse that I can hold my Serendipity charges, casting a ton of other heals: Prayer of Mending, Circle of Healing, Renew, Heal, Sanctuary, PW: Shield, etc. I can save my Serendipity stacks if I know a big thing is coming up: a pulse AoE where I'll need to heal the raid, or a big tank heal for a tank swap or timed ability.
Why Infusion Sucks, In Comparison
Infusion procs by Holy Shock crit so it's a chance instead of a guaranteed proc. I'm cool with that. An Infusion procced with a non crit, every 6 seconds would be ridiculously overpowered. So how it procs isn't a problem IMHO.
The problem is how we spend it. See that list of stuff up there, stuff I can do as a priest while saving my Serendipity stack for something I know is coming up? Here's the list for what paladins can do that heals that won't eat my IoL buff: Holy Radiance. Oh, I might be able to spend Holy Power, if I have three stacks.
So, we have one--maybe two--spell choice(s) we may or may not even want to use before our IoL proc gets eaten up by our next cast. There's no choice here. I cast Holy Shock (and half the time I'm already casting my next heal before my UI even registers I've gotten IoL), my next heal is short. Whether or not I need it that fast. Whether or not I want to use it right away, it gets spent. The only way I can save it is to stop casting, which I think we can all agree is a terrible idea.
Did I mention that a holy priest, after choosing to hold her Serendipity proc a few seconds, can also choose to use it on either a group heal or a single-target heal? Paladins get... a single target heal.
I don't think I have the answers. I'm not suggesting that IoL should be more limited in how we can spend the proc because that would just make the spell clunky. And Serendipity is arguably a minor part of priest healing, while Infusion of Light is a major component of paladin healing (though I could argue that it's only major because paladin spells are so limited).
Managing daybreak procs and Infusion is also a bit clunky. Using Daybreak to Holy Shock back-to-back can waste an Infusion proc if you double-crit. But critting Shock > hasted Divine Heal (.7-.8 seconds at our gear level) > Holy Shock is awkward to time. I can't chain cast it near as fast as I should. It breaks my casting rhythm (though I realize how personal and small of a complaint this is. My big beef is the no-choice-to-spend-or-hold argument above).
But that's why I don't like Infusion of Light. There's no choice in using it. It happens, then it gets spent almost immediately. It's a passive RNG-based HPS boost. I can't hold it to save a tank, I can't rely on it to proc when I really really want it to. And even if it does proc at a good time there was no Enlynn-saving-the-tank. It was a lucky proc, is all. Unlike Serendipity where a well-placed hasted GH could actually be smart play.