Here's how a level 90 mage thinks about the new talents
The level 90 talents (invocation, rune of might, mana shild) are now available for testing on beta since 3 days; and as I was testing them in
a) daily questing environment
b) heroic dungeon environment
c) duelling environment
as frost and fire specced mainly.
I felt like that they are not in the place they should be.
Rune of Might is fun, but having a cast time to set them and only granting 10% spell power seems weak and greatly cuts into the fluid playstyle of the scorch casting/running fire mage as well as the positioning-changing-kiting frost mage.
--> Rune of might would be better if the rune's (you can set two at a time) would stack (granting 20% spellpower, but u can not move out of it, and cast 2x new runes at another place takes a lot of time) Alternatively, let the rune cast be at least instant, so that you can play more fluently.
Mana shild; the passive 8% spell are not too bad, but the fact that you need to absorp dmg to gain the (up to) 30% spell power buff is very situational. Normally, in a raiding environment, as a caster I try to avoid dmg as much as possible, and only 15sec duration seems a little bit short. PvP wise, the talent is fine, but PvE wise, I'd rather see the duration/cd increased (maybe with a glyph, doubleing the cd (to 50sec) of mana shild, but also increasing the duration up to 30sec (from 15))
Invocation; Is in 90% of all situations the best talent, because it grants 25% spell power, lasting 40 sec... the talent is perfectly fine (even overpowered) for arcane specc, because you just stand spamming AB and then evocate. Repeat , profit. But for fire and frost, this talent really sucks, because you don't want to evokate every 36sec as fire mage (ruins your ignites/proccs on the target)... As fire and frost, even as arcane, having to evokate every 36 sec makes your whole playstyle disrupted, it's like you have to take a 4 sec pause every 40 sec, what greatly reduces my fun. As for suggestions, maybe let invocation/evocation being used while moving, or maybe change the talent that when evocation is successfully completed once, your next evocation cast time is reduced by 50% (as long as you still have the 25% spell buff). This would greatly improve the buff awareness and would not feel like a stop-and-go round based playstyle any more.
Each of the three is designed to have its strengths and its weaknesses. You then choose the "best" talent whose strengths match up with the situation you will find yourself in. Thats the design goal. You do a pretty good job of identifying each talents strengths and weaknesses, but for some reason you want to buff each one to not have a weakness, so that they're all usable in every situation.
You'd never take rune of might on a movement fight, because the way it works is completely unfriendly to movement. Thats the nature of the talent though, and you shouldn't expect it to get buffed to be movement friendly. You'd use rune of might on something like ultraxion where you can turret the whole time.
After pointing out how bad rune of might is for PvP, you then talk about mana shield, and completely ignore the fact that it solves all your PvP problems with rune of might, AND gives you an additional absorb for survivability. In PvE of course you avoid damage, but a lot of the time, you're going to get hit, and its unavoidable. Any fight with pulsing AoE damage makes mana shield an excellent choice. You'd take this for something like Beth'tilac or maybe Yor'sahj.
Invocation needs to have a higher spell power return to balance out the fact that it comes with a "dead time" where you're not dpsing the boss. The increased spellpower offsets the downtime, and ideally should balance out with the other two talents. Arcane's highest dps is no longer AB spam, and its concievable that you wont want to burn through all your mana to immediately evocate, and instead will try to seek somewhat of a balance between high mana consumption dps and evocation downtime. All that depends on the specific dps/mana cost numbers they settle on. Regardless, as you've properly identified, the downside here is stationary channeling and dps downtime. But again, this is part of the talent, and working around its limitations is the point. You wouldn't take invocation for something where you move a lot like Blackhorn, but it'd be amazing for situations with damage buffs, or that require burst such as spine or Hagara maybe.
In short, everything has to have a weakness to be a meaningful choice. Don't expect them to get rid of spells weaknesses because they wont (and they shouldn't). If one spell is ALWAYS bad, or ALWAYS the best, then that's an issue. Matching the talent to the fight however is part of being a good player.