Tim Geithner is a safe pick, and I think the market celebrated the news today because it eliminated some uncertainty. He's been intimately involved in crisis management this year as head of the NY Fed, so he brings continuity to the process, and at times like this that's probably better than holding out the prospect of a brand-new player.
He seems to be a consensus player, but he doesn't have original or out of the ordinary views on why we got into this mess to begin with. We don't know if he believes that lower taxes are better than higher taxes, for example. Presumably he shares many views with Larry Summers, which is probably good, since Summers was in favor of free trade and a strong dollar.
He's not likely to do anything radically different than what we've seen out of the Paulson camp this year. But he's also not likely to bring new thinking to the table either. It would be nice if he came out strongly in support of a strong dollar and a more stable Fed monetary policy designed to give the dollar's value priority above economic growth. But at least he is not a neophyte and thus shouldn't prove too destabilizing.