Heal also suffers more drawbacks that people tend to look out upon. Taking ignite against a heal literally cuts the heal in half, and you'll still be taking damage afterwards, (For example, at level 1, being ignited deals a total of 70 damage. Your heal will only heal you for 47.5 health with the debuff. This continues to scale upwards in the direction of Ignite.) However, there is no reduction to the amount of shielding you receive from a Barrier.
I feel the only changes Barrier needs right now is perhaps a slight duration increase, maybe from 2 to 3 seconds. That gives Barrier much more significance in the long term of a team fight, and also allows more counter play to damage-over-time abilities. Although in the end, Barrier is the spell you take when wanting to block burst damage, not as a general defensive spell like Heal.