The right ways to use healing meters:
Apples and Apples
Comparing apples to apples simply means comparing like to like. Some classes work differently to others, you see, and as a result, will perform better or worse on healing meters. A fine example of this is discipline priests. Because of their heavy focus on absorbs in their healing, they are likely to do well on meters. The reason for this is the effectiveness of absorbs as a way to heal. If there is an absorb on a player, and they are taking damage, the absorb will be knocked off that player before they even start losing health and needing actual healing. The absorb is always "used" first, always the most effective heal.
So if you were to compare a discipline priest to, say, a restoration shaman, whose spells are at their most efficient when they're cast on players at lower health, the discipline priest would likely come out on top every time, all things being equal. The exception to this would be if the fight was not going well, and people were spending a lot of time at low health, allowing the shaman's mastery to really shine. When things are going well, pretty much anyone else will struggle to outheal a discipline priest. Mistweaver monks are similarly omnipresent at the tops of meters, while resto druids join shaman at the lower end.Apples and Oranges
What I'm not saying here, though, is that any comparison between healers of different classes is completely invalid. There are occasions where healing meters can show you players who aren't pulling their weight. Say, for example, you have a discipline priest, a resto shaman and a resto druid in your raid. The resto shaman and the resto druid are roughly even, but the disc priest is doing barely half their healing. Something is wrong. Either the priest is very low-geared compared to the others, or they're having some other issues. That's an occasion when it's OK to look at meters and wonder what's up.
Do keep an eye on overhealing rather than just looking at raw throughput, as overhealing is just mana wasted. It's only useful healing that matters, although that being said, some classes will overheal way more than others simply by merit of their mechanics. Skada displays useful healing and overhealing really well, in the Total Healing mode.Self-Assessment
One of the best ways to use healing meters, though, is to compare like for like. If you're in a raid, or in LFR, and you're playing a resto shaman, and you're being massively out-healed by another resto shaman, this is when healing meters come into play. Look at the meters, look at World of Logs, look at what they're doing differently to you. Maybe they're using Chain Heal more, maybe they're dropping their Healing Stream Totem more efficiently than you are, or maybe they're doing a way better job of keeping Riptide up on multiple targets.
Look at the numbers they're getting out of each spell, and if your meter of choice allows it, look at how many casts they have of certain spells, how many ticks, how many crits, how much overhealing. And then look at the same numbers for your output. Is there something different? Maybe you could do better. It's important to remember that the optimal way to do this is in the same fight, and you should also check the other player's gear, glyphs, talents and so on if possible.
For example, looking at the above, I'm on the right, another resto shammy with the same glyphs and talents but better gear is on the left. You can see some major differences between us, for example, my healing stream totem did double the healing, implying that I'm dropping it more regularly. My riptide also did over 1m more healing, again implying that I'm using it more. However, my Healing Rain is way behind. And the difference between our Healing Tide output is almost certainly gear-based, the numbers are so close that we would have dropped it the same number of times.
If you're not in the same fight as another player of the same class and spec, you can always use World of Logs to find a high-performing player of your class and spec to compare yourself to. Given the huge number of parses on World of Logs, it's almost always going to be possible to find a similar one to your own. Do be sure to consider things like the length of the fight, the target healing, deaths, and the like, and try to find something as similar as you can. If you're comparing yourself to a healer who was a dedicated and focused tank healer, and your job was to look after the raid in general, your numbers, throughput and techniques are going to vary considerably!The wrong ways to use healing meters:
The worst way to use healing meters is by ignoring all of the above. Sure, healers in your teams may be underperforming, I'm not asserting for a moment that all healers should be immune to criticism at all times.
But don't use healing meters like DPS meters. This is the biggest mistake people make. Not only are healing classes very different, but healing styles are too. Going back to our earlier example, discipline is very strong, both because it offers a lot of DPS, and because of its reliance on absorbs. Put a discipline priest with a resto shaman, and your disco priest will be ahead 90% of the time. But when the proverbial really hits the fan, shaman are incredible clutch healers, with massive throughput cooldowns. They're a burst healer, and they're really good at it. They do have better maintenance with 5.4's buffs to Chain Heal, but what disc does well and what resto shaman do well varies a lot. And maybe your resto shaman is happy to let her disco co-heal take all the glory and wait for things to go wrong. Maybe that's just their style.
So don't use healing meters without consideration for classes. But also don't use them without consideration for roles and fights. Certain healers will do better when the raid is spread, certain healers will do better when they're stacked tight, some will deal really well with raid-wide AoE, some will do better with spot-healing. Some will be better tank healers, while some will be better at raid healing. Consider all this when you're looking at the meters and considering who to bring to your raid. What's more, healers can do more than just heal. Maybe that lower healer dispelled every single thing that needed it, or interrupted every cast of a certain spell, or CC'd when they needed to.
Now, I don't support the notion that just as long as nobody dies, your healers did fine. There's always room for improvement, and self-criticism makes for a better player. Inevitably, there are healers, based on class or individual, that will outperform others on certain fights, but none so bad that you shouldn't bring them along based on meters. I mean, sure, if you're Method or Blood Legion, maybe those tiny margins matter. But let's be honest. You're not.