Ret pally is a very strong class that can top heals / damage in a game under the right conditions, I've done it quite a few times playing with 9 Ally Jaja's on my team... /cry
The only difference in abilities between 20 and 29 is a dot, doesn't do much damage but handy for keeping rogues from restealth, glyphs like judgement slow + kings buff, and divine protection which is 40% magic damage reduction. I'd say ret pally is one of the better classes to make a vet because of this, plus crusader is op. Make sure you pimp it out with an onyx claymore regardless of faction.