This leads to the question of what to balance towards
Many people seem to consider that Barbarians, Bards, Rangers, Rogues, Favored Souls, Spirit Shamans, Warlocks, Warmages, Beguilers, and Binders are the general sorts of classes to balance around, since they do what they say on the tin without spilling over into other roles or tapping into ULTIMATE COSMIC POWER without the itty bitty living space.
Personally, I consider that we want to balance toward two things:
1) We want every class when built in a straight forward manner to be roughly tier 3, in that as you say, we want each class to be able to fulfill their primary role, perhaps be ok in a secondary role or two, but not be able to go it alone. That is to say, we want each class to be able to shine as part of a group, but not outshine the group or be able to take on challenges intended for a group solo.
2) We want a group of 4 PC's to find a suitable degree of challenge against a predictable set of foes, so that novice DMs have some guidelines regarding how to design challenges.
We also want to have a goal of being able to open up space for playing recognizable fantasy archetypes, but without continual power creep. That means, when we extend classes via chargen resources like Feats, we want to avoid strengthening builds that are already strong, and we want to avoid redundant stackable resources. In general, that means we want to avoid "there is more than one way to do things". 3.5 on the other hand continually reinforced builds that were already strong, while continually providing multiple routes to achieve the same result. These individual routes were often not overpowered on their own, but when players cherry picked the best elements of race, feats, new skills, skill tricks, PrCs, and equipment the results were generally undesirable. In general, the 3.5 designers seemed to never think about what they were actually doing or the implications of it.
And we could go on and on for pages in this vein, discussing how 3.5 designers never seemed to understand basic things just how hard it is to design a good feat, churning out tons of feats just because they were short, or that when you created a spell you were in fact adding a class ability to a class and so you ought to be really hesitant to do this without good cause. And on and on and on. 3e D&D was victimized by its own failure to rein in its own obvious extensibility, ultimately resulting in a train wreck and the over compensation of 4e.
Question: Would you say that a feat like Law Devotion, which lets the Paladin use Turning attempts to hit things better, is better balanced than Sacred Healing, considering that there are other classes with a generally similar mechanic (ie, the Barbarian) and hitting things is something the Paladin does anyways?
I would generally feel that if you spend a feat to give you some additional use of a class ability like 'turning undead', that it is probably reasonably balanced if it doesn't do more than what we'd expect of 1st level spell. In this case, Law Devotion is IMO better balanced than Sacred Healing. The appropriate comparison is with Divine Favor, which isn't a swift action and grants a smaller bonus but which on the other hand boosts both hit and damage. And maybe most importantly, Law Devotion is limited by 1/day which means you are effectively only gaining a single limited spell slot - not a number of slots that scales with charisma and which can be boosted by feats like Extra Turning which wasn't created with the idea that it was a spell slot booster in mind. If it wasn't limited to 1/day, then IMO the bonus it offers is simply too large and the feat too much of a no brainer. Whereas, spending a limited resource like a feat to gain a 1/day ability is an obvious tradeoff.