It implies:
* Extreme negativity: There's a sense of hopelessness and despair.
* A sense of impending disaster: The speaker believes something terrible is about to happen.
* Lack of optimism: There's no hope or possibility of good things happening.
Examples:
* "The news was full of doom and gloom about the economy."
* "Despite the company's recent successes, the CEO is spreading doom and gloom about the future."
* "Stop with the doom and gloom! There's always a silver lining."
The phrase is often used humorously to point out someone's excessive pessimism, or to contrast a positive viewpoint with a negative one.