This post won't really teach or contribute any relevant info about priests (so if you are here for that, no need to keep reading, the only somewhat relevant part is the weaknesses about PoH that I bring up), it's mostly a cause of me lying sick in bed and wanting to rant a bit about all the players who frequently claim that the PoH spam we did in (25 man) 5.1 is just as mindless as the atonement spam we do in (10 man) 5.2. I don't claim that the playstyle we had in 5.1 was very interesting, it really wasn't, I just claim that it was significantly more challenging and interesting than our current playstyle.
Initially it should just be said that comparing PoH to atonement as a whole is a bit missleading. You still casted holy fire and penance (offensively) in 5.0 as well, if only for evengalism/archangel stacks. The spells to really compare are really the 2 fillers, smite and PoH. Worth noting that one can argue that the rotation outside of the fillers is slightly more interesting in 5.2 (cds are comparatively stronger to fillers, pw:s has a lower cost, binding heal glyph is cool), but generally you are using the same spells then as now (just different amounts/times, e.g. more smite/bh/pom, less gh/poh). PW:S weaving (currently the most effective way to deal with sustained aoe damage) being far more interesting than 5.1 PoH spam is a very good point in favor of 5.2, but this gets nerfed in 5.3.
The first difference is targetting. PoH has a group targetting system. Yes, there are addons out there that'll chose the "ideal" target for you, but this isn't necessarily always the right choice. E.g. sometimes you'd rather PoH 3 targets and include the squishy, than 4 durable ones. It gives you another thing to think about when positioning yourself, you can use yourself as a "PoH beacon". You need to consider and set the groups up properly before the fight,
this doesn't always mean the melee in one grp and the ranged in one, there are many fights where you'd rather have a "healer/melee/ranged beacon" in each group instead.
However the most important thing is that your addon does nothing to tell you what group is the more important or how many targets you actually will hit with PoH. Generally you'd rather SS all 5 people in 3 groups twice and let your reactive healers/partners take care of the other groups than to SS groups where you only will hit 3/5 players.
Compared to this, smite is targetted at the boss, period. On some fights there are adds present but there's still usually an obvious targetting priority for you (mobs with +% dmg->normal dps prio). If you play with ToF sniping procs for this with smite, while still not making your smite completely wasted due to the add dying, can present a slight challenge. However this is rarely so predictable that this is a better option than just solacing/holy firing it in ToF range.
Difference #2. Cast time. PoH's long cast time allows and even encourages you to predict damage, and start casting it before it went out making it hit just as the damage arrived. A longer cast time also forces you to pay more attention to your other healers actions to ensure that you don't end up doing only overhealing. With the guaranteed DA application in 5.0 these two things hardly had that much impact, but still worth noting.
That being said, the main challenge that a longer cast time presents is the fight and any mechanics forcing you to move. You need to ensure that you can stand still for over two seconds when you start casting a PoH (or that time is wasted), this both means that you need to take more care in your positioning and pay more attention to the incoming mechanics.
Compared to this smite has close to a one sec cast time, there's very few mechanics where you need to move so quickly that you can't simply finish the cast and then move.
Difference #3. Spell synergy. PoH works is affected by/works differently with SS/IF. Proper use of PoH doesn't only mean using this specific spell properly, it also means knowing when you use your IF and SS with it. There's also the matter of haste "breakpoints" for PoH/SS. Not only can you gear around the haste required to get one extra PoH in during SS, you also need to consider how this amount changes if you factor in borrowed time and even PI, while also changing your playstyle/spell usage to accomodate for them giving an extra PoH (or not). PoH also doesn't provide evengalism and forces you to weave in other spells at the right times to ensure archangel uptime.
Smite on the other hand has the holy fire debuff. This does bring smite closer to greater heals hps but doesn't really change the output ranking of all our spells or our spell usage. You should not delay your hf/solace to maximize the amount of smites during the debuff. You will use smite as a low/medium hps filler when holy fire/solace and penance is on cd, with or without the holy fire debuff up. In very specific situations the absence might force you to use a different filler, but 99% of the time you will use the same spells regardless of this debuff. Something worth noting that did give the 5.1 PoH playstyle slightly less complexity is that it basically eliminated the usage of PoM (except during movement), unlike 5.2 where the spell has far more use (and the charge system makes it an interesting spell).
The effort to usefulness ratio of this post turned out to be even lower than I expected, I guess I'm really bored atm.
Tldr version: Nerf atonement to the ground!