Why no 5/5 Concussion talent? 
The answer to this one gets a bit Theorycrafty™ … but it basically comes down to the fact that the talent only boosts the BASE damage of Lighting, Chain Lightning and Shock spells by +1% per talent point. It does NOTHING to any spell power you might have equipped.
So if you’ve got a spell that does 100 damage … 5/5 Concussion would increase that amount to 100*1.05=105 damage.
But if you downrank to conserve mana, the base spell is going to be doing less damage … and the boost from 5/5 Concussion goes down with it. So if you’re instead using a spell that does 40 damage (to conserve mana) … 5/5 Concussion would increase that amount to 40*1.05=42 damage … which is kind of lackluster in terms of return on investment (see: rounding errors). 
If you’ve got +100 spell power boosting those example spells I just demonstrated above, the math adds up like so:
- 100*1.00+100=200 damage (0/5 Concussion)
- 100*1.05+100=205 damage (5/5 Concussion)
- 40*1.00+100=140 damage (0/5 Concussion)
- 40*1.05+100=142 damage (5/5 Concussion)
Point being that you’re not seeing returns on investment that would be possible with this order of operations in the math:
- (100+100)*1.05=210 damage (5/5 Concussion)
- (40+100)*1.05=154 damage (5/5 Concussion)
So if you’re planning to be in the habit of downranking your spellcasting in order to improve your sustain/endurance, the Concussion talent isn’t as “awesome” as it might at first appear.
Lightning Bolt (rank 3) @ Level 14 is the earliest you can obtain 5/5 Concussion and at that rank the spell does 45-54 Nature damage. 5/5 Concussion is adding an additional 2.25-2.7 damage per spellcast:
- 45.00 - 54.00 Nature damage (0/5 Concussion)
- 47.25 - 56.70 Nature damage (5/5 Concussion)
Spending 5 talent points to add ~2.5 damage per Lightning Bolt (rank 3) cast definitely falls into the category of … marginal benefits … that can be difficult to notice in actual gameplay (see: rounding errors).
The Concussion talent CAN “make a (meaningful) difference” … but you effectively need to be using max ranks on the spells that can benefit from the talent in order to get a useful return on investment into it. It’s just that you need to jump through a lot of hoops in order to get it (max spell rank, critical hit, Elemental Fury boost to crit damage) or even really be able to FEEL the difference (more damage per cast = fewer times casting needed).
But in a build where the idea is to cast only a few times before switching to melee, the priority on max damage possible per spell cast drops to lower rungs and can be (somewhat safely) discarded in favor of other options. 
Also, just as an aside, if you’re using your Fire Totems … 3/3 Call of Flame can actually have a slightly better return on investment than 5/5 Concussion would in terms of overall damage throughput for comparable mana cost. 
Offer not valid in Molten Core (of course). 
The real Secret Sauce™ for this build is actually the combination of:
- 3/3 Eye of the Storm
- 3/3 Elemental Devastation
- 5/5 Flurry
This combination of talents makes the “swing role” between caster and melee much more productive overall, allowing both casting and melee roles to “feed” off each other in complimentary ways that wind up ending fights faster. 
5/5 Shield Specialization (+5% Block chance) and 5/5 Anticipation (+5% Dodge chance) are included to mitigate incoming damage while in melee, improving sustain/endurance/longevity. Obviously, Shield Specialization is useless if planning to wield a 2h weapon (Staff or similar). However Pallytime215 demonstrated that 1h+Shield is net better for endurance/sustain than 2h+Earthbind Kiting (which requires enough room to work), including an in-game demo and calculation (1h 8m 30s video LINK) when normalizing to “effective HP” to determine comparative survival rates.
TL;DR … 1h+Shield is net “better” than 2h+Earthbind Kiting for survival and sustain over the long haul.
But if you’re still determined to wield a 2h, use 5/5 Ancestral Knowledge instead of 5/5 Shield Specialization (easy swap).
Just recognize that +5% Block chance AND +5% Dodge chance combined amounts to an Avoidance mitigation equivalent to +100 Defense (kind of 
). So for anyone chasing the 440 Defense cap, Shield Specialization plus Anticipation makes that task a LOT easier (only need +40 Defense on gear)! 
One last thing that I want to mention for this build is the choice for Shaman Weapon Enchant (Rockbiter, Flametongue, Frostbrand, Windfury). I have been convinced by this Frosthance Shaman guide on youtube that for a 31/20/0 Elemental Flurry Frostbrand Shaman the best option is going to be Frostbrand Weapon … NOT Windfury Weapon.
Long story short(er) is that Frostbrand Weapon gets 10% of your spell power added to damage on procs, but unlike all of the other options … Frostbrand (when it procs) has a chance to critically hit. NONE of the other options will critically hit.
- Rockbiter … adds Melee Attack Power and Threat, deals no elemental damage
- Flametongue … adds Fire damage, but does not spell crit
- Frostbiter … proc chance for Frost damage and slow, can critically hit spell damage
- Windfury … proc chance for extra hits with extra melee attack power
Where this synergizes best with the 31/20/0 Elemental Flurry Frostbrand Shaman build plan is thanks to Elemental Fury … so the Frostbiter procs do x2 damage (after boosting for spell power) instead of x1.5 damage on proc. The proc rate seems to be 9.0 PPM according to the sources I was able to find online, so even a fast 1.30 weapon speed dagger can do all kinds of “interesting things” (spell pushback to casters, damage+slow procs 
to foil runners, etc.). It just seems like a really good 1h+Shield combo that leverages the investment in Elemental+Enhancement for the greatest return yield when not hard casting. 
