Most people from Blizzard's World of Warcraft Official Forums seem to agree that the retribution tree is the best for paladins who want an easy way to level up early on. Blizzard Poster Eyonix pitched in two cents on the matter, saying retribution certainly provides more damage per second, but protection paladins can train fast too.

It all started when a level 19 paladin asked what skills are the best for a pally who can't spend too much time leveling and needs an alternative. Eyonix replied:

Retribution is the more common leveling build, and it certainly provides more dps than the other two trees (especially true next patch). Most of my friends who have leveled quickly did so as a protection paladin. You just have to know what you're doing and how to optimize when leveling using the protection tree.

Looks like the next patch will indeed buff paladins up a bit. Time will tell if we'll see them more often in raids, but it won't be surprising when their numbers grow.