The economic case against George W. Bush is straightforward, and Matthew Yglesias nails it in three sentences.
Kerry can be maddeningly vague at times, but in a broad sense one knows where he stands — the government should spend more money in a variety of areas in order to try and solve various problems and tax rates should be set at a level adequate to matching those expenditures. An alternative philosophy would hold that tax rates should go down and then spending reduced to bring expenditure in line with revenue. Bush, though, doesn’t hold this latter belief.
There is more to it than that, obviously, but when you boil it all down to essentials that’s basically what we’re left with.
UPDATE: Matt (briefly) responds to my post and makes more good points here.