The conditions are in some respects similar to Libya but not the same. I will also talk about the geopolitical aspects to this at the end.
ANTI AIR DEFENCE
Libya had an antiquated air defence. The decades long sanctions as well as feeling safe from external attack due to Libya cosying up to the West by renouncing WMD's meant it didn't rush to upgrade it's air defence once the sanctions were lifted.
Syrian air defence, though like Libya relies mostly on old Soviet era SAMs and Aircraft is much more dense and robust. It's been quite well maintained and they do have some modern SAM's recently bought from Russia. Another problems is Syria has a lot of it's SAM networks near civilian areas so if NATO were to carry out SEAD missions, collateral damage could be high.
The rebels controlled large swathes of territory in Libya. This degraded Libya's integration of it's air defence. Syria for now has no such problems. Syrian rebels battle for towns and cities but that's as far as it goes for them.
UN SUPPORT
During the Libyan crisis the UN was authorised to create a no fly zone to protect the civilians. There were no veto's from the 5 Permanent Members and thus legal under international law. NATO used this opportunity for regime change and with air support the rebels eventually managed to overthrow and defeat Gadaffi. Russian and Chinese economic interests were curtailed.
As for Syria, Russia and China both don't want a 'Libyan' scenario were the UN R2P is used for regime change. And so both continue to veto UN resolutions and want 'both' sides to come to the negotiating table and Kofi Annan's peace plan to be implemented. On the US side, Bashar Al Assad no longer has the authority to rule Syria and so want him removed.
REALPOLITIK
Obama and more importantly the Pentagon don't really want to get involved in another civil war and in fact just another war. They realise the Syrian situation is far more complex than in Libya and need Russian and Chinese support in Afghanistan and against Iran. Also the rebels seems fragmented and there are Jihadi elements which the US (though not Saudi Arabia and Qatar) seems to be wary of. The current Baathist regime in Syria, though outside the US orbit of influence is a case of better the devil you know.
GEOPOLITICS
Despite the dangers of getting involved military in Syria, there are advantages should the regime there fall. Iran and Hezbollah would lose a major ally. Russia could possibly lose it's only foothold in the Mediterranean (the Tartus naval base).
