Theoretically, both direct and circumstantial evidence should carry approximately the same weight in court, but in reality direct evidence seems to win out over circumstantial. I can look up at the sky and see water coming out of the clouds, accompanied by lightning and thunder. I can show a radar image of thunderstorms in the area. I can videotape the rain as it falls on my yard. Those pieces of direct evidence should convince any jury that it did indeed rain yesterday.
But I can also walk out on my porch and see wet grass. I can also see a tree with a scorch mark. I can hear my neighbors complain about a storm damaging their roof. That's all circumstantial evidence, but it should also be enough to convince a jury that a storm occurred yesterday.
The problem is, a defense attorney for Mother Nature can argue that the water on my grass was caused by a sprinkler system. The tree was not struck by lightning; it just broke in half on its own. The neighbors could have been discussing a storm that happened weeks ago, or you may have misunderstood their (inadmissable) conversation completely. *That's* the problem with circumstantial evidence. Mother Nature's defense attorney can't argue much about a videotape or radar readings, but he can plant a boatload of reasonable doubt in the jury's mind with circumstantial evidence.
As with the recent Casey Anthony case, you can't rely on the "if it walks like a duck, quacks like a duck and looks like a duck, it's probably a duck" theory. A jury wants to see physical evidence of a duck, and it wouldn't hurt to call Daffy Duck and Donald Duck to the stand to testify about the nature of ducks, walking and quacking-wise.