Arguably that it was insufficiently progressive. Of course, that is not what one heard during the Bush presidency when that tax code was made both more regressive as well as less able to collect revenues, playing a non-trivial role in turning a US budget surplus into a massive deficit, now being used by Republicans in the US Senate to push for cuts in all kinds of social safety net programs. Because of their firm opposition there will be no going back to it, and when Clinton got it put in place in the early 90s, they totally opposed it, forecasting it would lead to a massive recession. It did not, but they took control of the Congress on such claims.
I note for anybody who thinks that the Clinton code was some sort of "anti-rich" pile of socialistic redistributionism that the top marginal income tax rate in it was still well below what was in place after the first round of Reagan tax cuts in the early 1980s, when that rate was still at 50%. The really sharp cut in the top rate only came in 1986 with the tax simplification.