First magic has become unpredictable. In the old days you built your power dice into your list. Now it's based on a roll of the dice. Now for most armies, this isn't a big deal, but there are a few armies, namely the undead, that rely on their magic. For those armies, life has become a bit of a challenge. When you need a strong magic phase, be ready to roll a small number of dice. My Vampires have seen this costing them dearly on the table. Their units are largely mediocre because they can be replaced with a few spells. Now rolling a 2 for the winds of magic at a key point will cost them the game. Any ability to affect the number of dice will come in handy so Dark Elves, Villitch the Curseling and certain gear have become far more effective in helping you get a strong magic phase off.
Spells have become more potent...or should I say devestating? The low to mid spells are decent and handy but the upper spells become just beastly. Honestly I feel this was why power dice became variable from turn to turn. It balances the spells in the rule book and big spells should be seen as a bonus to fire off instead of something to be counted on. Of course the magic found in army books are a bit different. Army-specific spells are generally not as insanely destructive as the new rulebook spells, but also are easier to cast.
Of course all isn't fire and the death of your foes. Miscasts have changed to become far more dangerous. The spell is guranteed to go off, but you have the potential of paying dearly for it. You can lose spells, wizard levels (even losing the ability to cast at all) and possibly even have your caster explode taking nearby models with him.
Magic is still a powerful aspect of the game, but I don't think building an army around the magic phase would be effective. No one should leave home without a caster or two for defense and the potential to destroy a huge hammer unit if things work out the right way but armies that win with their magic won't be winning often. Some armies have access to some fun tricks to help them maximize the magic phase. Empire wizards can be built cheaply and have access to every school of magic. High Elves have some spells of their own that are force multipliers as well as access to some tricks to affect dice and casting rolls. Chaos Sorcerers are more durable then most to the point that they shouldn't fear combat as much as most casters and Tzeentch makes getting spells off easier. Lizardmen probably have the most going for them with the potential to shrug off a miscast. The best thing is to look at your army and decide where magic can fit best and how many points you want to dedicate toward the dark arts.