Dave's relatively inoffensive. Vocally, at least. Nobody's ever going to call him a great singer, but nobody's going so blurt out "yee gods" and switch the song off, either. And while I've never been a big Megadeth fan, they have their moments of greatness, and Dave's singing fits what they do perfectly.
And in comparing him to Ulrich, Dave has always been a perfectionist. Lars, along with Hammet, often suffers from dontgiveafuckitis. He brings the ridicule on himself. I'm a big Metallica fan, and mostly indifferent to Megadeth, but I'll make the same jokes myself. And insofar as live performances go, there's just no comparison. Megadeth is an exercise in precision while Metallica plays with the wild abandon of a Jimmy Buffet cover band.