You need to think about three different concepts as a healer. They are:
Throughput: Amount of healing you can put out per second
Mana Longevity: How much total mana you have access to over the fight
Mana Efficiency: How much healing one point of mana buys you
Stacking haste is the fastest way to increase your throughput, but it doesn't boost your mana efficiency or longevity at all. The amount of healing you can do in a fight is the same with 0 haste or 1000 haste. Having 1000 haste just lets you do that healing faster. If you are running out of mana in fights already, there is no reason to stack haste.
If you stack int, you are increasing your longevity. You have more mana over the fight, so the total healing you can do increases. Ignoring the minuscule amount of spell crit that int provides, it won't increase how much healing each spell does though. So there's no change in throughput or mana efficiency. If you aren't running out of mana, there is no reason to stack intellect.
When you stack spell power and crit, you're increasing the effect of your heals. In other words, each point of mana buys you more healing than it did before. This is an increase to mana efficiency. (In the case of crit, extra crits improve your holy concentration uptime, which also gives longevity, but I will ignore that for now in the interest of simplicity.) Spell power and crit are the only way to actually gain mana efficiency. It helps both when you are running out of mana (though not as much as int) and when you need more throughput (though not as much as haste).
If you want to increase both the total healing you can do in a fight and your throughput, you can either stack just spell power, or you can stack a mixture of haste and intellect. It turns out that because of the coefficient returns on spell power, you get better overall returns from spell power than you do from the mix of haste and int. You should never have both haste and intellect gems in your gear-- that's suboptimal. You either want haste and spell power (at high gear levels) or spell power and int (at low gear levels).
Additionally, spell power gives better efficiency returns than crit. So the possible ranking of stats from a gemming perspective has these constraints:
spell power > crit
spell power > haste or spell power > int (or both)
Some common stat rankings:
haste > spell power > crit > int (max throughput)
int > spell power > crit > haste (max mana longevity)
spell power > crit > int > haste (max mana efficiency)