1) No, it's not like Iraq. One was a relatively stable, if brutally autocratic regime that the US administration decided it wanted to oust, and believed it could replace with a friendly government (at minimal cost, no less). The other is a state involved in a very active, bloody civil war. If y'all didn't want a government that might intervene to stop ongoing mass slaughter, you should have made a stink when Samantha fucking Powers became a close advisor to the presumptive president of the United States.
2) The British and French governments have been the hawks on Syria, generally being restrained by the US.
3) If you read the TPM story, the main caveat isn't whether the Syrian government has chemical weapons or whether they were used, or which side used them, but whether Assad personally ordered their deployment. Fine. So? If Assad didn't order the attacks, but allowed them to happen and is protecting the officers who gave the orders, he's given his sanction. If a US president tried that, we'd recognize it right away as a bullshit attempt to avoid responsibility.
4) The use of chemical weapons, even with the public "red line" rhetoric from the White House, is not the sole factor in US policymaking, and probably not even the most important one. To reiterate, there's a civil war happening in a state that borders three or four US allies (depending on how you want to count Iraq), with more allies (Saudi Arabia and some EU countries) demanding action and already getting involved themselves. Hopefully nobody gets any crazy ideas about a ground invasion and occupation, but this was never something that the US could avoid any involvement whatsoever.
Worth reading
this NYer piece from May on Syria. Take it with a grain of salt because Filkins strikes me as tilted towards the rebel cause, but it's an interesting snapshot of the pressures inside and outside the US government on Obama to act.