Tips and advice for paladins

Paladin stats by paladin type
Primary stats for each paladin type in alphabetical order, and not necessarily in order of importance; that is left up to the player based upon their goal and playstyle.

Secondary stats can be optionally obtained after your primary stats have been met, if you choose to do so.

Healing and non-Retribution DPS
Spell crit rating is the primary stat for a healing set, but for DPS it is largely useless. Seal of Righteousness and Seal of Vengeance cannot crit. Their Judgments can crit, but if there is a tradeoff between spell damage and spell crit, spell damage should be taken for a dps set. And any build having Holy Shock will include (as a matter of course) Divine Favor (paladin ability) which gives a guaranteed crit to your next heal or Holy Shock, and can improve dps in that manner.

For a real solid healing build (or any other pally build) spirit should be ignored. There may be times in dungeon runs where you have time that you're not spamming spells where spirit could help you regen your mana. But when you're not spamming spells is hardly the time that you really need the mana regen that you want from spirit. Furthermore, spirit gives us no side bonuses that some priests and druids have such as a bonus to healing.

In a raid, there is usually more than one healer. One for the tank and one for the group. If you are the healer of the tank and continually find yourself assisting in the group healing it is usually one of two problems. Either the tank is not generating enough aggro or the DPS group is not watching what they are doing and dealing too much DPS, which thus pulls the aggro off of the tank. This usually leads to a group wipe.

Protection
Since patch 3.0.2, Paladins no longer need added Spell Power from equipment, not even on a weapon. Paladins share the same tanking equipment with Warriors and Death Knights. The primary aggro stat is Strength, although paladins still benefit from Spell Power. Through the talented passive skill Guarded by the Light, Spell Power is scaled to Strength (patch 3.2.2), providing all the Spell Power that is needed. Changing your old Spell Power weapon to a Warrior tanking weapon is suggested as much of the aggro is build up from weapon damage, Hammer of the Righteous and other abilities. Most paladins will notice an increase in aggro buildup when switching to a DPS tanking weapon, which comes along with the improved survivability.

You may find that hit rating (as of patch 2.3) is helpful when tanking to reduce the number of Righteous Defense resists, or perhaps you may find that the number of resists you see is acceptable due to not having to use it as frequently as the next guy. Hit rating bonuses can also be helpful, considering the fact that we have to actually hit with our melee weapon in order for your favorite seal (of Corruption or of Vengeance) to proc. Unlike warriors, druids, and death knights, we also get threat from Holy Shield, assuming they are already attacking us, and Consecration, assuming they are in the vicinity.

Protection paladins do NOT need intellect, mana regeneration, or spirit on their tanking gear. A tanking paladin should expect to either take enough damage that the heals that are cast on them will give them enough mana regen. to finish the battle, or dodge/parry/block enough hits to generate mana by Blessing of Sanctuary (paladin ability).

Retribution
Retribution paladins are by far the best talent specialization for single-target soloing and they are equally viable in raids as well. With the right build, Retribution paladins can kill mobs quicker than mages, warlocks, or rogues, though downtime is increased. A good Retribution paladin should put 2 points into Protection, as you'll want Seals of the Pure. 3 points into Holy for Judgements of the Pure is also extremely useful. Vindication is nice for PvP (3/3 for 15% reduction in all stats, including Stamina), and for solo / small group PvE, but it is mostly useless in raids as raid encounters tend to be immune to it. Every ability that improves Seal of the Crusader is absolutely essential, and Seal of Command is just a given - DPS is non-existant without it, and Crusader Strike is a necessity too. Aside from those points, simply throw the rest wherever you want - this talent spec is kind of loose.

The primary stats you'll be wanting to obtain are, in order, Strength, Hit Rating 8%, Expertise and Critical Strike Rating above 25%. After you have met those requirements you will want to get Haste, and Mastery. The key to retribution paladins is to bring down enemies quickly.

Plate, or Mail under level 40, should be your armor of choice because of the Strength bonus for wearing all plate armor pieces.

Retribution paladins use large, two-handed weapons, such as 2h-swords and -axes. A very good starting point for any Retribution paladin is to take up Blacksmithing. Blacksmiths can make their own armor, which can be especially useful for leveling characters who can make armor on par with dungeon drops.

Aside from this, you should just experiment and do what works, it takes a lot of practice, a unique gear, stat, and spec combination, and a little luck to get Retribution nailed down.