I did a little bit of work on mana over the past few days1 and there are a few important conclusions that distinguish Cataclysm from Wrath. One is that Replenishment is no longer the ultra-important buff that it used to be. Of course you don't need a spreadsheet to figure this out, the calculation is pretty simple. If you have 100k mana and it gives you 1% of your mana per 10 seconds, then it is giving you 100 mana every second. In a five minute fight that's 30k mana. We all want 30k more mana to spend over five minutes, but we can manage without it more easily than we can manage without some other buffs. Of course you may have more than 100k mana - and if you are a discipline priest who is raiding you really ought to have well over 100k mana, but replenishment is now only about 60% to 100% better than the Mp5 buff given by Blessing of Might and Mana Stream Totam2. It's a very powerful but probably not absolutely necessary buff.
Of course getting replenishment isn't that hard. Six different classes can supply it with one of their specs, so most raids should be able to access it. A significantly more powerful mana buff, however, is available to exactly one spec of one class.
Mana Tide totem increases Spirit by 350% for 12 seconds every three minutes. The 12 second duration can be increased by 40% by a talent to 16.8 seconds. That gives it an uptime of around 9.3%. Getting 350% more of a stat 9.3% of the time is pretty similar to getting 32.6% more of that stat all the time, if the gains can be averaged out. Of course this is actually the worst cast scenario for the totem. You get a Mana Tide every three minutes at least, but in a five minute fight, you probably get two, which is one every two and a half minutes. This would increase Mana Tide to a 39.2% buff to spirit. It's all a little odd given that they said there would be no buffs to spirit in the game.
So how much mana does this actually provide? In a five minute fight, using my current gear, it provides me with a little over 50.5k mana. Druids, paladins and shamans all get a little less than that, in the 45k-48k range. Holy priests get around 84k mana from it - making mana tide about a 25% boost to the mana available to a Holy Priest over the five minutes.
Of course this kind of bonus dramatically alters the value of your stats. If you have a shaman to drop the totem for you then spirit is about 60% better than intellect of paladins and shamans, nearly on par with intellect for druids and discipline priests and a whopping 100% better for holy priests at generating mana. Without it, those numbers decline dramatically with spirit being only 20% better than intellect for paladins and shamans and similarly worse for the others. This kind of difference should significantly affect gemming choices. As a discipline priest, I'm much more likely to match a socket bonus with a blue gem if the spirit is as good as Int at giving me mana than if it is only two-thirds.
And of course mana tide totem stacks. I don't think it literally stacks, but they can be staggered to increase the uptime. More restoration shamans means more mana.
Currently the most powerful mana buff in the game by far is available from one spec of one class only, and stacks if you have more of that spec of that class. This buff situation does not seem appropriate.
1. I built myself a little Mana Calculator to play with. It's not very user friendly and the estimates for the mana gains for Water Shield are pulled out of a hat - two issues which I may correct in the future if I am interested in doing so.
2. In the first tier of Wrath raiding replenishment was about two and a half times as good as the Mp5 Buff. By the end it was four to five times as good.