Ho ho, my friend, there are a LOT of differences. Not necessarily a bad thing, though. There are some things DA2 did better (talent/spell trees) and some things they did worse (enemy wave attacks). Just bear in mind DA2 is a different beast and you'll be fine.
However, I would say that mage, built correctly, is still probably the most powerful class. So, nice to know some things don't change.
Yes, Arcane Warrior is gone. I'm not mourning it. It was way overpowered, and shouldn't be a common spec anyway given its acquisition. Shapeshifter is gone too. Mana Alteration might seem a loss, but there are noticeably fewer enemy mages in DA2 than in Origins. When they do pop up, they're lethal, but they're conspicuously few and far between.
On the other hand, Force Mage is completely awesome! It is crowd control beyond your wildest dreams. The best spec for a mage (I'd say "best in the whole game" if it weren't for Assassin, and even that's a bit neck-and-neck). It's great for yanking a whole bunch of enemies together and then dropping a powerful AOE on them, which pretty much sounds like what you're going for. Build this tree ASAP. Fist of the Maker and Telekinetic Burst aren't so great, so don't bother upgrading them, but you will need to get them to unlock everything else, which is pretty much all golden. Go nuts on it.
For the damage side of your CC/Damage sandwich, the elemental tree is all rather tasty. Chain Lightning in the Primal Tree is also very good. Between these you'll be hitting a lot of enemies extremely hard. The obvious downside is that they're AOEs, so your companions might get caught in friendly-fire when you play at higher difficulties. You could make sure to boost their magic/elemental resistance (Elemental Aegis, for example, is great for warriors), or you could simply field an all-ranged party (which is a pretty good call generally - your ranged companions all rock).
For your second spec I have to say that Blood Mage is rather underwhelming. If you want to go that route, then bear in mind to build your constitution, but even so it's not the powerhouse spec it was in Origins (it was better initially, but then one of the patches nerfed it). Spirit Healer is okay, but it doesn't sound like it fits with what you really want to do with this build. In short, I probably wouldn't waste my points on a second spec tree. Force Mage and Elemental are the key ones in this build. The bottom half of the Creation tree is great (Haste and Heroic Aura are both very nice). On the other hand, Walking Bomb was simply made for a Force Mage and is both brutal and hilarious - even though the Spirit tree is otherwise largely underwhelming.
Entropy seems a good enough tree, but to be honest, I've never really build very deeply into it, myself. Like, ever. Maybe that's just my oversight. But in any case, it doesn't seem to fit terribly well with your build. And in any case, you will get a companion who seems to suit it, if you're set on getting some mileage out of it.
Another thing to remember about DA2 combat is Cross Class Combos (CCCs). In DA2, each class has some abilities/spells which can put enemies in a particular state, and other abilities/spells which exploit the other states for lots of extra damage. Mage has BRITTLE, Rogue has DISORIENTATE and Warrior has STAGGER. As a mage with Winter's Blast (and possibly Dessicate too?) you'll be causing a good few Brittles, so make sure you give your allies abilities which exploit them, and make sure you set this in the tactics. Here's a reference chart:
http://dragonage.wik...oss-class_combo
One final thing: I want to make a special note about enemy rogues (Rage demons count here too). Newbies HATE them. Quite understandably. They turn themselves invisible and then suddenly appear behind one of your party members dealing MASSIVE amounts of damage in a backstab, which is quite often an instakill. They're quite the little buggers. However, they are manageable when you know how...
The important thing to note is that rogues do not simply teleport. So when they have gone invisible, they still have to cross the battlefield to get behind your party member. The thing is, some spells/abilities have the power to knock them back out of 'invisibility', and thus abort the backstab. Fireball is one. Winter's Blast is another (though this one is rather difficult to pull off, since it's range is small and centered on you). Bursting Arrow is another (one for your ranged rogues there). And if Force Mage didn't have enough to recommend it already, Pull of the Abyss works here too. As soon as you see a rogue go invisible, instantly fire off one of these at the spot where you last saw him (or slightly in front - they will have moved in the time it takes you to fire the spell) and ta-dah! Then lay the hurt on them! All the hurt!
Good luck, and enjoy.