The trouble is, between Holy and Retribution, there is a world of difference in how to play. Granted, most of your spells will be pretty much the same, but the playing style is completely different. As much as Shockadins will protest, the truth is, Holy is a support spec. It is terrible for questing and the best thing to do is find a partner who can kill things for you. Despite the improvements to Holy Shock and the spell damage included with healing gear, the experience pales in comparison to pure DPS classes or specs. This is fine. The only real beef I have is in the disparity in play experience and the nagging feeling that, at the heart of it all, Paladins are glorified sidekicks.
I suppose that for the most part, this is true for all hybrid classes, but the offensive experience diminishes when speccing Holy as opposed to the other two hybrid classes (and to an extent, Priests). For one thing, Holy Paladins have 0 spammable offensive options. None. Nada. Everything works on your swing timer and an 8 second, 10 second, and 15 second cycle of Consecration, Judgement, and Holy Shock. Consecration actually plays a role in a Holy Paladin's spell cycle because of its 100% damage coefficient -- while it is a complete waste of mana for Retribution Paladins unless you're using Rank 1 to flush out Rogues. Anyway, the point is that when speccing Holy -- even as Shockadin -- you're always just waiting on a cycle that's pretty much rote.
When you spec Retribution, you remove the 15 second timer and insert a 6 second one for Crusader Strike. On paper, this doesn't look like much of a difference, but philosophically and practically, it moves everything forward. More damage over a shorter period of time so the thrust, generally, is purely offensive. It takes a different mindset to play Retribution because if you're playing it right, your idea is to deal as much burst damage as possible. On the flip side, the new itemization for Retribution that came in Patch 2.4, as well as Blizzard's intention to create a shared loot table with Warriors, gimps Retribution healing to the point that it's almost impractical to heal.
This has bothered me somewhat not necessarily because it's difficult to adapt from one playing style to the other. I'm sure Druids have more of a headache seeing as how they're shapeshifters, and I'm sure it's wildly different playing walking firewood from an overweight chicken. It's just that, these past weeks, it's become extremely glaring to me how the Paladin class was designed from the ground up to be a support class. There's just no going around this fact. A vast majority of Paladin spells are support-oriented, and the glaring shortage of controlled offensive spells can make playing a Paladin frustrating.
I'll try to explain. When I play my Shaman, there's very little difference in playing style because I can easily shift from offense to heal support with minimal impact on performance. A Restoration Shaman has pretty much all the tools available to an Elemental Shaman, for example. In an Arena match, if all the Restoration Shaman's teammates are topped up, she can easily switch to throwing Lightning Bolts or Chain Lightning. Throw in an Earth Shock on an opponent's spellcast for good measure. The same applies to a 5-man instance or heck, I'll throw it out there, a raid. If everyone's topped up or near full, it's actually sometimes useful if the Shaman shifts to offense. And she'll even be effective at it. The only real difference is in the numbers, but the versatility is there even without having to re-spec.
On the other hand, a Holy Paladin whose teammates are all topped up is left to do precious little. Going on offense isn't even an option because healing Paladins running in to whack at an opponent with their tiny one-hander healing hammers just won't be doing anyone any good. Any Druid, whatever her spec, can spam Moonfire or Wrath (not quite sure why you'd want to, but you can). In PvP, Druids of whatever spec can cycle Cyclone on opponents. I guess what I'm trying to say is that a Paladin specced in a particular fashion is pigeon-holed into that role more than any other hybrid class.
The one offensive spell available to Paladins of all specs is Judgement, and it isn't even a purely offensive spell. It's a utility spell that Paladins must choose to use as a damage dealing effect or an auxiliary set-up every 10 seconds (8 if talented). This doesn't leave us a lot of options. All other spells are situational (Holy Wrath, Turn Evil, Cleanse, etc.) or preparatory buffs (Blessings). Auras activate the global cooldown so aura dancing isn't even wise, particularly in Arena matches because it'll lock you out of key spells for 1.5 seconds.
Now that I'm Holy, when I'm soloing my daily quests and I get ganked, I really don't even bother fighting back. Sure, I can last long enough, but I'll almost always eventually die anyway so I just roll over to save all of us some time. If I were Protection-specced, at least I'd enjoy it if a Rogue or Warrior tried to gank me. It's a completely different attitude change from when I'm specced Retribution, where I actually feel like I have more of a fighting chance. When I'm playing my Shaman, I don't have to exercise such a massive paradigm shift.
I suppose this is the drawback of being touted as the 'defensive hybrid'. More than anything, playing the class' different specs will reinforce the idea that the Paladin is all about support. Cycling through the specs will expose the class design for what it is -- a support class. More often than not, everything a Paladin does is geared towards helping others. A Paladin works best with others. That's just how the class is designed. Even Paladin tanks, fantastic as they are (don't let anyone tell you otherwise!), need healer support. I know all tanks do, but you understand what I'm trying to get at.
Blessings, auras, heals -- everything is designed for support. Even most Seals are designed for support when unleashed as a Judgement. In many ways this is a good thing, but it's a playing attitude that all Paladins must inculcate. We are at our best when we are part of a group. As a player who mostly enjoys solo play, this is a hard lesson to learn. I like Retribution because it allows me a modicum of freedom to play solo. The other two specs, not as much (yes, I know farming with Prot is fun). And despite switching roles, as hybrid classes are wont to do and likely to cause dissociative identity disorder, this is the underlying philosophy that Paladins must follow in order to play the class right. As good as Paladins are on their own, we shine brightest when we have friends.