It's really more the heightened power of the minority caucus of the Senate which has recently radically expanded what were previously relatively minor veto points in the path of legislation[1]. It's not quite a liberum veto [2] yet but it has some of the pathologies.
[1] http://upload.wikimedia.org/wikipedia/commons/thumb/7/79/Clo...
I think Congress used to be more unruly, unpredictable, and less partisan, so as things got more cut and dried over the years, vetos just stop actually occurring as a practical matter, unless there are points to score politically by making the president go through the motions.