Pretty much the same as Obama's on the other side.
Something fresh, with no visible baggage, and pretty much unknown.
Most voters are looking for their own version of a Democrat or Republican (one reason you should never pay attention to "generic" polls asking if you want a Democrat or Republican for an office). They can project their own thoughts onto candidates that don't have a lot of info out yet.
We'll know more about Thompson--and Obama for that matter--when the primaries start weeding things out.