Though it still exists as a talent, Demonic Portal is actually an example of removing weaknesses in a way that harms class differentiation. Going back to BC, one of the core differences between the two pure offensive casters, warlock and mage, was that warlocks were far more durable and "tanky," while mages were frail but had more mobility and escape tools (Nova+Blink being central to that). Giving an instant teleport/escape ability like Demonic Portal to warlocks did a great deal to blur that distinction, and the class's design might have gone in a quite different direction without it.