As Noam Scheiber notes, it's tough for people to explain why they respect Antonin Scalia ("even if he's conservative, he's still brilliant") but not Clarence Thomas ("he's... uh... I think...") without sounding a wee bit racist. But the easy answer here is that Scalia believes in stare decisis (the notion that one ought to respect precedent), and Thomas does not. That makes Thomas a wild card, a radical, and one scary dude.

(According to David Garrow, though, some of Thomas' decisions betray a more muted tone. See United States v. Lopez, where Thomas says he'd be willing to interpret the Commerce Clause in a manner that reflected recent history: "Although I might be willing to return to the original understanding [of the Commerce Clause], I recognize that many believe it is too late in the day to undertake a fundamental reexamination of the past sixty years." Of course, many believe is not quite the same thing as I believe, yeah? And mightn't he feel more emboldened if a few more judges felt the way he did? Hence the worry at nominating another Clarence Thomas-type judge!)