I haven't seen much discussion of mana issues. Converting most gear SP to Int would suggest a very large jump in total mana at high gear levels. Blizz has also implied that PvE DPS should not normally be concerned about mana issues (Arcane Mage is an obvious exception).
To make mana even marginally an issue (and if it is no issue at all, why keep the stat?) they can do some combination of
- Lower the Int->Mana conversion (currently 15)
- Significantly increase Base Mana (and therefore all spell costs)
- Remove or weaken mana-regen abilities (Dreamstate, Meditation)
- Increase the cost of individual spells
- General mechanic changes (no more 5SR, in-combat means no spirit regen, even when not casting).
It isn't clear where they will go with this. It is a potential concern. At least in a five-man you may have no regen from other classes (no Replenishment, BoW, etc), and it is at least plausible that you'd stay in combat for long periods of time (perhaps six minutes in ToC5 or HoR type encounters). A Moonkin who is OoM has no native regen other than Innervate.
Does anyone know more about where they are going with this? Mana is a very number-crunchy issue, and at this point I think they are more concerned with look and feel, with the assumption that if they get that right, the numbers are fairly easy to tweak.