Heresy we're reluctant to name

A lot of blogspace has been devoted to Rob Bell's universalism, along with charges of heresy. Yet why are my fellow-Reformed friends so reluctant to call Arminianism heresy, when it is a greater error and does more damage to the doctrines of grace? To say that man is sovereign over salvation and that the atonement is not sufficient to keep believers secure is a terrible heresy, yet we smile and agree to disagree. I think Rob Bell may at least have a higher view of the atonement than most Arminians.