1) There is (at least) visual evidence confirmed by several eye witnesses that chemical weapons were used.
2) This time the people are speaking out against unilateral involvement including cautioning against a missile strike.
While I will agree that it does seem as though this country tends to get involved in military conflicts everywhere else around the world except in African where so many humanitarian atrocities take place on continuous bases, the reason it appears that way, however, is because the media rarely covers such issues on a regular basis that is until it affects America's interest (i.e., Black Hawk Down). Nevertheless, I will admit that the rationale given to strike against Syria does mirror the excuses given for invading Iraq. This is why I'm very much against America's involvement beyond "leveling the playing field" - arm the rebels as Russia is arming the Syrian Army - only in this case it has to be done in multilaterally not unilaterally.