What exactly makes this class a Defender?
In a way, to me the significant difference between a Defender and a Controller is that the Defender makes himself a target. He provides "incentives" for the enemy to attack him, and he doesn't generally "hide" from his enemy.
That's for example why marking only triggers when the defender is not part of the attack. And why the ranged abilities of Defenders (if any) are usually very limited in range. He has to get close to his enemies so they actually have the option of targeting the defender. If they don't, because the defender can defend from range unmolested, the defender is just a controller with lots of extra healing surges and hit points.
So, Arcane Ward should not protect the Abjurer, I think or at least work somewhat differently.
1) The Arcane Ward is probably too strong if it grants resistances. Temporary hit points should be better. Say, at the start of each of its turns, the target gains 3+[SECONDARY ABILITY] temporary hit points.
2) More powers should have the Arcane Ward Destruction Mechanic, with the idea they perform somewhat better if the Abjurer can use it then when not.
3) If the Abjurer himself gets hit, he loses his Arcane Ward. Maybe for each hit, he loses one ward.
2 and 3) encourage enemies to focus on the Abjurer instead of his allies. Enemies don't want the Abjurer to deal extra damage because the ward is active, and focusing on the Abjurer means they get rid of all those wards that protect the allies as well.
If you want to keep him at range, you're naturally running into a conflict with how a Defender has to operate. If you can't guarantee that enemies "stick" around the Defender and keep targeting him, you must make him pay the cost of defending in other ways.
For example:
Protective Runes - Standard
You exert your inner reserves to draw magical runes of protection over your ally.
Range: Melee Touch
Target: One ally
Effect: Spend a healing surge but regain no hit points. Your ally gains temporary hit points equal to your healing surge value. Until the end of his next turn, the ally also gains a +2 power bonus to defense and saving throws.
Dimensional Guarding - Standard
You erect a dimensional warding around your enemy, that allows you to hinder his attacks, but opening you up for his attacks in turn.
Range: Ranged 10
Attack: Intelligence vs Fortitude
Hit: 1d8+INT force damage and the target is marked.
Effect: Until the end of your next turn, the enemy can make melee attacks against you as if you were adjacent to him.