“What exactly did President Obama gain by conceding the planned missile defense system”

Well if you assume that Russia & the USA are inevitably & enternaly locked in combat then nothing except maybe saving some money.

However if you say that there is no real clash of interests now, America having nothing to gain by the ethnic cleansing of Ossetia then there is an awful lot to gain.

Russia has more reason not to want Iran to have nukes than the US has but America’s picking a fight at all opportinities has made them sympathetic to Iran.