@Finn dAbuzz,
Geithner's a very centrist, middle-of-the-road man in his politics - and he's got the aura of experience/expertise due to his previous posts. I suppose that for a centrist Republican, he would be a fair enough fit (or at least not one to actively block) - a better fit than for someone like Bernie Sanders, actually.
That just leaves the tax issue. I think there's obviously a lot of Democrats who would've been indignant about it if it had been about a Republican, but didn't care much now; just like there were a lot of Republicans who would have shrugged it off in a Bush appointee but now were indignant. So that covers most of the Senate. But just like there was a couple of guys on the very left who were as indignant about it in a moderate Democrat as they would have been about a Republican, maybe there was a similar couple of Senators in the political center who recognized that they would have shrugged it off in a Bush appointee and decided to shrug it off for an Obama one too.