Reforging
Reforging began in 4.0. It is a great way to reach the stats you are looking to stack or supplement that the gear that does not give from its base stats.
For a holy paladin you want to reforge in this priority:
Spirit > Haste > Crit
Mastery is really useless. You will never reforge to gain this stat.
You want haste. This was scaled down and now to get a minimal of around a 2 sec Holy Light/Divine Light, you need tons of it.
Spirit is what you need for regeneration. You still pretty much end up spamming during fights. Spirit is the only stat that will give you mana back.
Crit is not really important. The only reason it is on the list is because it is waaaay better than reforging for mastery. It does not hurt, but it is not your target stat at the moment.
You need to be careful with stat stacking. Too much haste with using the wrong heals will give you the outcome of a oom paladin. Too much spirit, then your heals will not heal high enough on your target that you have to cast extra heals to get the HP where it needs to be. There is some straight up nothing but haste blue pieces that you would just not reforge unless you are really struggling for mana. Being in 5mans, it is hard to scale whether or not you are where you need to be at because nine times out of ten, its a pug 5man from a que, and there are alot of mistakes you end up healing. This will not normally happen in a raid unless your guild/pug consists of bad players and you are trying to heal alot of mistakes.
The best advice I can give from the knowledge I have is, if the person will not stop messing up and taking extra damage.... let them die. The tank and better players would be the more wise place to put your heals.