Thoughts on Syria: at the moment, it looks more like Clinton bombing the Sudan in 1998 than Iraq III.
If the question is strictly one of justice, then some kind of intervention is demanded, if we have some capability to prevent the worst abuses of the Syrian government (and the rebels). That part's not a complicated question. What the 'some kind of intervention' is will be.
I have gradually come to oppose these types of actions, in spite of the moral clarity of the case. The problem is that most of the western world, and the United States in particular, is unprepared or unwilling to stomach the realities of intervention. Successful interventions take a lot of time--decades, not months or years; they require serious and sustained involvement--the good of the people you seek to help has to become, in some way, your good. As Tocqueville noted, though, the characteristic of America is an inability to focus on a problem for long enough to actually solve it. If it will be an issue of concern for, at best, a few months, then it's better to avoid acting.
But this is a particular type of objection, not at all different from refusing to permit a civil rights march because it might incite violence. That is to say: it is entirely up to the general public to decide what is important to it. While it may, pragmatically and in the moment, make sense to concede to the reality of the political situation, we should be aware that the entire system functions only because of a fundamental decision to be unjust. American interventions don't work because the American people don't care about the rest of the world (this is not a problem unique to America, though we are its apotheosis); it is their own intransigence that sabotages the action, not the wrongness or impracticality of the action itself.