Five factors seem to be at play. First, this spring Georgia asked to join NATO. Despite Washington's unequivocal support for Tbilisi, European states expressed reservations about accepting Georgia before it resolved its border conflicts with Russia. The re-firing of the conflict will surely increase the potency of that concern and push Georgia's NATO membership beyond the horizon.
Second, Russia wants to retain its domination of the European natural gas market. Europe's energy dependence on Russia is growing from day to day, and this endows Moscow with significant income and political clout. A large part of the natural gas that Russia markets to Europe is actually from Central Asia, and Moscow coerces those states to sell it to Russia at half the price for which it then resells it to Europe. In recent months, Central Asian states have explored circumventing Russia and transporting their gas resources directly to Europe via Georgia. The present conflict clearly upsets these plans.
Third, the Kremlin made it clear that if Washington recognized the independence of Kosovo (as it did), Moscow would recognize and support the independence of the secessionist regions in the Caucasus. Russia is extremely vulnerable to ethnic conflict (remember Chechnya and friends?) and did not want the Kosovo precedent on the table.
Fourth, Moscow wants to foil US plans to deploy ballistic missile shields in Eastern Europe. Threatening a close ally of the US gets the message to Washington.
Fifth, following the installation of Dmitri Medvedev as president of Russia, in-fighting in the Kremlin seems to be at play, and Moscow's disproportionate response to Tbilisi may be influenced by this.
When the peddlers of moral equivalence or its not our problem since its so far away rear their head, ask if they would like to trade places with the Georgians or perhaps live in a Europe held hostage to Russian supplies of natural gas. Oh wait, the US being held to 70% oil imports does not phase them unless they are filling up so the problems of Europe or Georgia would not enter their empty heads.