If the Voice Director and the Voice Actor both understand the material and know which direction to go with then it will turn out good. This happens very rarely, and it has to be both of them. Examples of a good dub with this combination would be Gundam Wing, Read or Die TV, and Cowboy Bebop. If its just one of the two it will probably turn out decent at best.
If you have a good Voice Director but a cast that doesn't understand the material, you'll get a dub where the cast is trying their hardest to give a voice like what the director regurgitated into them on what the character should be but the cast isn't quite sure what they are doing and caring is another thing as well, so it just doesn't turn out right.
If you have a good cast but a Voice Director that doesn't understand the material pretty much the cast will be trying to do what they think the character should be but within the limits on what the Voice Director says is alright to do. There's also the voice director picking VA's for the wrong roles and such. The Gurren Lagann dub is a pretty good example, the cast knew what they wanted the characters to be and how they should sound but the director didn't really seem to understand the material and while the dub was alright, it just wasn't that great.
A good Voice Director (depending on the distribution company like Bandai) is usually more common than a Voice Actor that understands the material. Moreover because a Voice Actor usually doesn't watch the anime they dub before they dub, or they do watch it but don't become "absorbed" into it. Its not like they don't want to, moreover they don't have the time to. Crispun Freeman or Greg Ayres are good examples of voice actors that give the time of day to watch and understand the anime before they do what they do. Although Ayres he kinda of does a character like how he wants it to be not how they should be.