Back in the first half of the Bush Administration, while Republicans controlled both the House and Senate, Democrats frequenty invoked the '60/40' filibuster rule in the Senate to block Republican sponsored legislation.
Most people these days think it takes 60 votes in the Senate to pass anything. That is incorrect. If Senators who are against a bill want to hold it up, they can filibuster - basically taking over the Senate floor with a non-stop verbal diatribe of gibberish. As long as they keep talking, they can hold the floor and prevent any action from being taken on the legistlation. A vote can be taken of the members of the Senate to cut off a filibuster. To succeed, it must have 60 vote in favor of cut off. That is where 60/40 comes from. The vote on the legislation itself just requires a simple majority, but to get to that vote, you have to have 60.
Democrats used this tactic several times against the Bush Administration and Congressional Republicans. They called it their patriotic duty to exercise 60/40. When Republicans in the Senate started talking about rewriting the rules to eliminate the 60/40 roadblock, Democrats screamed bloody murder - they said any such attempt would be 'the nuclear option', and promised everything short of secession to block any change to the rules.
Fast forward to the elections of 2008. Democrats have held the House and Senate since 2006. Obama is elected. Republicans find themselves in the exact same position Democrats were in a few years before. They use 60/40 to block legislation in the Senate, just like the Democrats did. Democrats scream bloody murder again, this time because the rule is being used against them, instead of for them.
Faced with Republican control of the House, and a smaller majority in the Senate, Democrats are pissed that Republicans can continue to use 60/40 to derail Obama's agenda.
Preparing for the next Congressional Session, every single Democrat remaining in the Senate have written a letter to Senate Majority Leader Harry Reid, calling on him to unilaterally change the Senate's operating rules to eliminate 60/40.
So, the 60/40 rule is great when it benefits Democrats, but the exact same rule is evil when it benefits Republicans.