While I believe there are sound policy, electability, and visionary rationales for preferring Obama over the other Democrats running, there's no mistaking that Obama has largely been a non-factor as a U.S. Senator.
For all practical purposes, Obama has served in the U.S. Senate for two years, the last year being devoted to his presidential campaign. And while the typical rejoinder to the lack of experience argument is that such non-Washington credentials accents rather than diminishes his potential for real success (especially given the bad experience we've been acustomed to these last many years), it should still be said that the U.S. Senate is, or is meant to be, a place where national debates take place, and in which the public can be educated and be given access to the democratic process. The Senate represents America's premier forum for hashing out the differences, conflicts and aspirations of the national polity through the gathering and participation of America's finest political minds and most respected national leaders and spokesmen. But Obama's brief, and rather quiet, stay in the Senate has not been an example of the best the Senate has to offer (although neither have the presences of Edwards or Clinton for that matter).