Lately I've been trialling some new developers, which I do periodically.

Every time I do, it makes me consider what's really important when assessing whether someone will be a good fit for a project.

Obviously experience with the technical stack used on the project is important, as is overall commercial experience.

Those and ability to communicate effectively are what I'd generally use to make a decision.

More recently I've incorporated attention to detail in the assessment.

I define attention to detail as ensuring that features are implemented with all requirements satisfied, including implicit ones.

Lack of attention to detail results in incomplete or incorrect solutions. The direct result of this is rework.

During a trial, it's rework for the reviewer and the developer. In business as usual, it's rework for these plus a tester and the affected customers.

These things affect morale and reputation. They multiply hours and cost required for a given feature.

While it's important for a developer to have strong experience in required technologies, I'd argue that a deficiency here will be less damaging than lack of attention to detail, and an inability to communicate effectively.

Assessing each of these areas equally is going to give you a better idea of the true effectiveness of any developer.