The graphics on it, first of all, are something to behold -- the Shaman reaches towards the sky as a small cloud forms above him, and then suddenly, a bolt of lightning crashes down through his body as a circular shock wave crackles around him, tossing all enemies back 20 yards. It's pretty key for an Elemental Shaman to have such an electric spell, and it's definitely something you'll be able to see from a distance: a Shaman pushing back a crowd of mobs or players with a bolt of lightning. Very cool.
In terms of numbers, Thunderstorm is not quite as impressive, but still respectable -- in in the current build (I think it's the current build -- the numbers are changing almost every day now), the spell does 1450 to 1656 Nature damage instantly to all enemies within 10 yards. Additionally, it will knock all enemies back 20 yards, and restore 5% Mana to the Shaman. All on a 45 second cooldown (faster than most trinkets, and generally useable almost every fight). That's a spell working on a number of different levels -- it adds a little bit to Shaman AoE, but the knockback and the Mana restore are the two more important parts.
We'll talk Mana restore first -- the mana pool has always been a problem for Shamans, and there's no question that a 5% restore on a 45 second cooldown helps out a lot (doesn't necessarily fix the problem, but it's a huge help for Elemental Shammies). 5% is a nice chunk of mana, and not only does it mean less downtime overall, but it means that in emergency situations, a Shaman can spit out Thunderstorm and call back enough mana for an extra heal or a shock and a Lightning Bolt. A talent like Unrelenting Storm will help you more with a mana pool, obviously, but using this when it comes up will help, too.
The other function of the spell, though, the knockback, is even more helpful. In addition to the spell pushback change, this will give Elemental Shammies more chances than ever to get casting without worrying about being hit. The spell originally started out at 20 yards, and then was nerfed down to just a few yards knockback, but since then it's been brought back up to 20 yards, and that's a good enough distance to give any Shaman getting beat up a few seconds to make enemies back off, and figure out whether it's time to keep casting or get out of there. Combine with Frost Shock (or Earthbind), and you've got more than a few seconds (though not forty-five -- you can't juggle enemies with Thunderstorm, as fun as that would be) to either get a few uninterrupted casts off, or run for the hills.
There's also a little bit of positioning strategy in Thunderstorm -- if there's a wall behind the enemy, they won't go back all the way. And of course like any other knockback spell, you've got to make sure the area is clear -- knocking back one enemy into another will only get you in more trouble. But it's a fun spell to use, and it adds a new element to being a caster -- knock enemies back, and figure out the best way to use the next few seconds it earns you until they close again to melee range.
I haven't used the talent in PvP, but I can imagine that there's a lot of fun to be had, tossing the opposite faction every which way. The mana and AoE there is mostly a bonus -- while you'll get lucky every once in a while, it's not likely that you'll kill anyone with the 1500+ damage coming off of Thunderstorm. It'll be much more useful in keeping large groups of players off of a flag or out of melee range with you (and of course, occasionally throwing precariously perched opponents off of cliffs).
So as you can see, it's much more of a utility spell than an actual damage spell -- even in raids and places where you're not actually in melee range with mobs or enemies, that mana restore will come in handy, and when you absolutely need to kick a mob off of a healer in the middle of an instance, the knockback might give the tank time to turn around and Intercept.
Elemental Shamans came out well with their 51-pointer. When I do get my Shaman up to 80 (odds are more and more likely every day that I'll go with my Hunter first), I'll be doing it with the Elemental tree, and Thunderstorm will get a hit almost every time the icon lights up.
Want to know more about all the new spells and talents in Wrath? Our Skill Mastery series has more lowdowns on the new skills you'll be able to use in Northrend. And if going Shaman is more your style, the WoW Insider Directory's Shaman page has everything you'd ever want to know -- and then some.