Spark of Hope & other mana reduction
Disclaimer: In contrast to many recent (classic) guides, Spirit of Hope does not drop from Novos the Summoner, it drops in Ulduar 10 from Kologarn.
If we look at the stats (42 mana reduction on spells + 100 spirit) this looks insanely good for disco priests. At a GCD of 1s on PW:S - discounting latency - it should result in 210 mp5 + 50% gain from Meditation for 100 spirit. Of course it all depends on the situation, but let that situation simply be very stressful.
For holy paladins Libram of Renewal is bis, so why isn't this bis for disco priests? Because other stuff is better! But why is Abacus better? Let's do some math:
Abacus is basically a free random Flash Heal every 45 seconds. Thats 695 mana saved devided by 9 = 77.2 mp5. Then you have to add the extra pump that you gain from 201 additional spellpower. Here it gets tricky, because the result depends on many factors like crit%, overhealing% and so on. It better be a lot though, because there is a lot of ground to cover. Even if the result is close, I would prefer the static advantage of mana saved over a dynamic one.
Why isn't the Idol of Awakening bis for resto druids either? Same logic applies, that applies to holy paladins. Last but not least, Totem of Forest Growth allows my shaman to cast 53 chain heals before going oom instead of 47 without. Other shamans may get different numbers, yet same logic applies.
The only answer that I came up with is this: In the endgame you don't have mana issues, you don't go oom, therefore you don't need such mana savers. Well, that is not the only answer. Another answer would be that it all depends: In some encounters certain items are bis, in other encounters it's others. This on the other hand would mean that all those neat lovely bis-lists are somewhat relative. In order to get the most out of your character you have to adjust.