For me, the holypriest is ideally:
- Flexible. Large arsenal, a spell fitting every situation
- Strong. No heal feels weak.
- Mana intensive. A holypriest can always spamcast large heals, and do silly high HPS... for about 20 seconds. Then oom.
Our spec strength is that we can go crazy with healing. It allows us to pull off stuff like Jhazrun did in Garrosh 10H. We don't need any overdrive button, we have one by default: Spam harder. This is also the spec weakness, because no other spec will burn though mana as fast as a holypriest. However, if we hold back a little, that's not a major problem. It can be, if even our affordable spells cost an eyeball, but that was mostly a Cata issue. Mastering the management of mana is by far the holypriest's greatest skillcheck. Related, I suck at it.
To get the most out of our mana, it is important to spend it right. This is where the spell selection comes in. At any point in time, there are 15 different heals I can cast. Which spell allows me to maximize healing versus risk, overhealing and mana consumption in a given situation? Making those decisions on the fly is what makes the holypriest incredibly interesting and fun to play. When you do it right, you feel very rewarded. When you do it wrong, things start to die because you ran OOM having to catch up more.
But strength should never been the issue of a holypriest. All spells should feel strong. It's all down to how you use them. Another reason to hate Heal to death.
For me, the holypriest is currently highly inflexible. Most of this is basically ruined by chakra. Half my toolkit is wrong to use. Half my toolkit is weaksauce.