First of all, I played DAO on a PS3, not a PC. So there might be a few differences I don't know about.
I prefer to spec my characters for the highest DPS possible, ending fights quickly rather than expending energy into healing and support. I ended up going for a lot of area effects spells, topping them with weakening countdown spells. So um, no healing spells people. That's for the wussy mages. I get my feel-goods when my mage character is doing 50% of the party damage on her own.
What I did:
Arcane - Mastery Tree
Primal - Lightning tree
Spirit - Death Tree
Entropy - Draining tree
Entropy - Sleep Tree
Also these entire specialization trees:
A typical fight earlier in the game was me casting Tempest and Death Cloud over crowds of enemies, slapping on a couple Walking Bombs (regular and Virulent), hitting the strongest enemy with Curse of Mortality, and then casting a Chain Lightning. That combination cleaned up pretty well. I played quite a few fights without party support this way.
Since I quickly got the Arcane Warrior Specialization, I didn't worry too much about defensive spells or force fields. I could put on armor and with a complete Arcane Warrior Spell tree (going for the passive effect Fade Shroud) you're pretty damn tough. Before I got the specialization, it wasn't hard to just stay out of the fray and I rarely died even without any defensive spells.
The most intense spell combination I found was Lightning Storm + Blizzard + Spell Might (creates Storm of the Century, which quadruples the area it effects and pretty much instantly kills everything in the area). You can actually split these spells across two mages, which was pretty good for the timing of the spells. Pile on top of that a spell like Curse of Mortality (Entropy - Draining) or Virulent Walking Bomb (Spirit - Death) and watch your enemies explode each other to death in seconds. When going through dungeons, I found I could set area effect spells in rooms before I went in, from the other side of the door, which was ridiculous and awesome. Fill a room with death without even alert an enemy.
The more you stay in one school of magic, the easier it is to get equipment bonuses for your spells, which sounds nice in theory, but in practice there wasn't really that much different helpful equipment in the game for mages. I recommend doing your specs without too much regard to schools.
Remember that you can spec 2 additional different mages in the game to support you.
What I'd do differently now that I've played the whole thing:
I wouldn't have completed the Blood Mage tree. You do get a bonus to constitution and spellpower for choosing it, but I wouldn't waste points unlocking the spells. They were cool, but I didn't use them a lot because it required me to be in a high-mana costing mode. Blood Control is cool in theory but it kind of messes up your fight when all your enemies become your allies. My game didn't process it very well and we all ended up standing around until the spell ended (this would make more sense in Hard or Nightmare difficulty, because you can still damage allies). Also, if you're finding yourself desperate in a fight, using your lifeforce to power spells doesn't work out that great.
I might have gone with the Entropy - Hexes tree because Death Hex stacks awesomely with Death Cloud from Entropy - Draining tree.
I'd consider nixing the Entropy - Sleep tree. The spells were really useful, but didn't deal a lot of damage. Sometimes it really helps to walk into a room and Sleep all the enemies, however. The spells in this tree stack on each other well, as you can Sleep/Horror or Sleep/Waking Nightmare for extra effectiveness. Alternatively I might have done the Creation - Glyphs because they have some nice stacking spells (Glyph of Paralysis + Glyph of Repulsion = Explosions). Explosions are GOOD.
Spirit - Mana Alteration tree might be good to get up to Spell Might (the first 3 spells). It didn't seem like there were enough fights with mages to really justify putting points into this, but Spell Might adds a lot to your spell power and is necessary for some spell combinations.
If you want to focus on the arcane warrior specialization (very fun, btw) pay attention to whether a spell can be cast with your sword out. It interrupts your fighting if your character is always adjusting weapons. Here's a page listing whether each spell can be cast with your sword out.
Note 2: it really matters what difficulty you are playing on.
Casual/Normal makes it easy to fill rooms with area effects spells with few repercussions. If you're playing on a difficulty that has friendly fire (Hard/Nightmare), choose spells that hit one or a few targets, not entire rooms.
Note 3: After your first play-through, any specializations you've unlocked stay unlocked. This changes what spells are available earlier in the game. Also, if you've got multiple characters going at once, spells unlocked for one character will be unlocked for all characters on your account.
Note 4: Most of the primal spells were annoying.
The Primal spell trees are as follows.
Fire spells, when combined with grease spills make long lasting fires. However, once you've done this combo, you and your party are now susceptible to the fire.
Earth spells were cool except the area effect spell Earthquake, which knocks down your whole party just as much as your enemies.
Cold spells can combine with Force Field or Stonefist or a critical hit to shatter enemies. This is cool. What is uncool is that Cone of Cold and Blizzard will freeze you party too, stopping your attacks as well as they stop the enemies'. This is so annoying. What I ended up doing is casting all the Cold spells myself, even though they were on a separate party mage. Letter her auto-cast caused trouble for everyone. (It's worth it if you're going to try and create a Storm of the Century).
Lightning spells rock. Also, they have much faster cooldowns than the Cold spells. And electrical spells also drain stamina. No problems there, except maybe that the Tempest spell is really loud . . .
Also, the equipment bonuses for the Primal tree are a little stupid, because you have to get a bonus for each type, Cold, Earth, Lighting, etc. rather than getting a bonus for the whole school. For instance, you can get gloves with +10% Cold damage, OR you can get gloves with +10% Spirit damage, which covers more spells.