I believe it's based on the speed of the spell (before talents to reduce it's cast time) and how much of a bonus you get from your +healing/damage.

Here's some examples for Pallys.

    * 10% One-Handed Seal of Righteousness
    * 12% Two-Handed Seal of Righteousness
    * 50% Judgement of Righteousness
    * 20% Seal of Command
    * 43% Judgement of Command
    * 33% Consecration
    * 43% Holy Shock
    * 43% Hammer of Wrath
    * 43% Exorcism
    * 19% Holy Wrath
    * 20% Holy Shield (5% per Block)


    * 71% Holy Light
    * 43% Flash of Light

So where Holy Light gets 71% of my healing (2.5 sec cast), Flash of Light (1.5 sec cast) only gets 43% of the bonus.  

Then, when you get to a certain level of +healing/damage.

(I'm pretty sure this is how it works, someone correct me and I'll go /wrist if I'm wrong).

Now when you get to a point of +healing/damage, these coefficents start to change and you get less of a bonus towards your spell per point.  I heard that healing after you get over 1800 (I'm like 1750ish in raid buffs),  it's like 4 +healing equals what you would normally get with 1 at a lower level.

Therefore, contiuning to stack your +healing/damage past a point isn't worth it, and you should trade out for Mp5/Intish gear, cause you're just not getting the benefit.

I was reading a post about a Paladin who had 2100 +healing and could do some amazing huge heals, but had to pot every fight because they didn't have the regen or the mana pool to support it.

Anyways, I think that's how it works, as it was explained to the US Americans about The Iraq.