First, Hi @FunSteve, and welcome to Fluther. Great question, and I appreciate that you want detailed discussion in the answers.
Syria is a far tougher problem than Libya was. In Libya, you had a united opposition against a dictator who was a pariah around the world due to his terrorist activities and seeming insanity. The entire Arab League was united against him and requested NATO to intervene and prevent a genocide. The UN voted to authorize military action to prevent genocide.
In Syria, the opposition has yet to coalesce into one cohesive voice. It’s this and that faction independently fighting Assad. As odious a dictator as Assad has been, he never blew up commercial airliners flying over Europe or any such fool things as that. He has the support of Shiia Iran and Hezbollah, and the Shiia majority in Iraq would oppose any Western intervention.
Sadly, it’s going to take even more carnage to change that, or the civil war will just have to play out to its bloody end. Assad is a member of a tiny faction of the Syrian population and is opposed by nearly the entire populace. Bit by bit, the army is abandoning him and turning their weapons on his forces. He’s doomed, but it looks as if he is determined to die fighting for everlasting power. Power must be an incredibly seductive mistress, because so many dictators do die fighting to keep it when they could cut a deal to live peacefully and in luxury in exile.