The best argument I've heard, that I agree with in a way, is that we should first focus on fixing our broken pre-college school system. I don't see why we couldn't do both at the same time but if we're forced to pick one or the other I'd agree that that is probably of greater importance. Though that's a separate debate :)
Just remember that I don't have kids in school nor in college, and taking my money to pay for more unfireable teachers is not "fixing".
Why else would he be floating proposals he knows can never pass, except to be winning cheap gains from the suckers who don't see it for what it is? A president proposing a huge program like free college for everyone without controlling Congress is basically talking to a wall.
Obama is trying to help the Democratic party with its stagnant voter base problem. The people who would regularly vote Democrat have moved far to the left of the Democratic mainline and are tired of seeing nothing come out of the Democratic party, meaning that they no longer have any motivation to vote for Democrats-- not to say they are voting for Republicans either.
Sure, you're going to say Obamacare came out of the Democracts-- but the left leaning segment of the Democratic base didn't want Obamacare, they wanted single payer, and Obamacare fell horrendously short of that. This election was a wakeup call for the Democrats, and indicated to them that they have lost their base to the more radical members of their party like Elizabeth Warren.
Thus, we see the Democrats attempt to show people that they still have big ideas, because they are incapable of showing people that they have big actions.
But good legislation does not just magically appear in Congress. Many government programs got a big boost from the President "calling for them"--national parks, for example, or the moon landings. These were not new ideas when Roosevelt or Kennedy spoke about them, but speaking about them helped generate the public momentum for Congress to act.
The U.S. was the first nation in the world to provide free, publicly funded education through high school. It was implemented locally, but it was an idea that was discussed and promoted nationally. The idea of free education is a powerful one in the U.S. culture.
And many community colleges are already heavily funded by state and local governments; making them totally free might not be as big a step as it seems on paper.
I could also argue that if 4 year colleges have even in part become so expensive because of all the subsidized money available that bringing subsidized money to the community college will have an "unexpected result" so expected that one could hardly call it "unexpected" anymore.