The big problem is that the WMDs are still out there, and in the hands of terrorists or terrorist sponsoring states.
That simply cannot stand, and every day we let it stand we are putting thousands, even millions of people at risk. Our hands are tied in Iraq right now, but we have to go after the WMDs ASAP.
Here's the deal. If Syria gets more chemical / biological weapons, that's not good, but it's really not any different than before. They're just getting more of what they already have a lot of. They know they'll be dealt with if they use them, and they don't want to be dealt with. The Syrian government is a secular, Baathist organization that only exports jihad. They don't inhale.
As far as Iran and North Korea getting nuclear weapons, that's the real devil incarnate. We should do everything in our power to keep those weapons out of the hands of religious fanatics and insane cultists. I think we'll be in the position someday of looking back and saying 'Why didn't we act when we could have prevented all of this'.