First off, you need not only to talent up to Improved Fire Nova for this to work (easily doable with 18 in elemental as the talent calculator link shows you) but you also really need the Glyph of Fire Nova. With these two combined, Fire Nova is available every three seconds, making it your fastest cooldown. With this set up, you can hit Fire Nova between every other ability you have: A SS - FN - LL - FN - ES - Maelstrom LB = FN rotation becomes feasible. As your gear improves you could weave in more Flame Shocks. However, while it's feasible to hit Fire Nova every three seconds, it's not recommended in most situations.
First off, it's a ridiculous mana suck. Fire Nova costs a beefy 22% of base mana. That's even more than Earth Shock, and you could theoretically cast it twice as much. Secondly, if there's not enough things in range, Fire Nova won't really add enough damage to make spamming it worthwhile. While Fire Nova is an instant spell and therefore benefits from Mental Quickness, I found myself actually noticing mana being an issue when I used the spell every time it was up.
However, in a lot of situations that's exactly what you will do. If you're like me you're running as many heroics as you can for emblems and a chance at some nice off spec gear (my intention is to have three full sets of T9.25 for each spec so that I can finally have full hybrid flexibility: I've already collected offspec weapons and yay a healing trinket -- finally a trinket with no spirit!) so you end up in ridiculous trash pull after ridiculous trash pull. Even with the new AoE cap there's literally nothing the demented pyromaniac in me likes more than seeing a tank gather up two or three trash pulls in one place. You can see some ridiculous damage there, boy. But be careful, because you can also see some ridiculous threat doing this.
Generally as enhancement I couple Fire Nova with Magma Totem, which admittedly increases the mana drain on big AoE pulls because I have to refresh magma totem every 20 seconds or so. Then again, since I'm enhancement I can usually count on Shamanistic Rage and Improved Stormstrike to refill the gas tank if I'm getting low. Using FN with Magma on a trash pull allows for roughly six uses of FN (six times 3 seconds = 18 seconds) before you'll have to renew the totem, and frankly, unless it's one of the new ICC five mans or the AotD wave of the Black Knight in ToTC there's almost no chance anything will survive to have to drop magma again anyway.
For elemental I can imagine this being a trifle annoying to sustain, but I've seen enough ele shamans running into drop the totem and then just dropping back a couple yards to cast to indicate to me that it's worth it. One of the annoyances (to me) of Fire Nova is that when you drop Fire Elemental for the big damage situations, the Fire Nova pulse continues to radiate from the base of his totem (I'm not talking his own, very similar AoE ability, which comes from him): you can end up in a situation where you're kiting a boss out of some horrible gunk or another on the ground (because as we know everything in WoW has to have the ability to make the floor try and kill you) and you stare forlornly back at your Fire Elemental totem entirely out of range for a Fire Nova shot of any kind.
I said before that in non-AoE situations you don't want to spam FN, and that's true, but that doesn't mean you don't want to use it at all, mind you. If everything else is on cooldown and Maestrom is still ticking up, feel free to drop a FN burst in there even on a single target. It's more DPS than standing there waiting for something else to cool down or light up, and it's more fun than waiting for a Windfury proc or a "YAY MAELSTROM LIGHTNING UP YOUR BUTT" to happen. Not that I don't love putting lightning up things' butts. I mean, who doesn't? What else would you do with it?
In the end the addition of Fire Nova has made raiding and instancing much more fun for me. Is it the perfect solution to shaman AoE woes? Well, I honestly can't say for sure for elemental (I'll leave that to Sacco and his elemental focused column when it comes) but it's been nothing but positive for me as a enhancement shaman and I've even enjoyed using it as restoration on long, boring fights where my mana never drops below 75%. The ability to do some meaningful AoE without sacrificing other totems for it which still uses a totem to do it was a stroke of genius all told.
Check out more strategies, tips and leveling guides for Shamans in Matthew Rossi's weekly class column: Totem Talk.