Holy has a 6 second cooldown lay on hands in holy shock; only slightly exaggerated . They still have hammer of justice to peel dps or stop heals. They can use pursuit of justice pretty effectively because of holy shock, making them good kiters. Denounce stopping criticals severely lowers the enemy teams dps if it's used effectively and not just to do mad deeps. Their word of glory being used more often is a small healing increase that adds up over the course of a match and let's a bit more burst healing be done. They also have a cleanse.
Holy pallys lose 3 seconds of cc and 700 damage on 3 individual players, the cc specifically is invaluable if used selectively on a group of casters and not solely for damage, but the execute factor of it is powerful too. They lose more burst aoe damage and a handy tool to get out stealthies from holy wrath. Prot also gains some free survivability from guarded by the light that holy doesn't get. Holy is very susceptible to interrupts on either denounce or flash heal if they try casting while holy shock is on cd, since all of their spells are holy they're left pretty useless for a while. Denounces damage isn't actually great, and can be healed through without much trouble and generally isn't enough to kill in one HoJ, although it does consistent damage over a long time which helps pressure, until you have to stop to heal a team mate, assuming you actually heal... A last semi-con, if you run into an affliction lock (lol) and aren't careful with cleanse you can be silenced for even longer than you usally are, locks curse of enfeeblement also lessens your denounces effectiveness.
Holy is decent support but I definitely prefer prot for team oriented play, and even in one vs one situations I'd have a hard time recommending holy.
