I don't seem to hear much about any other countries possible involvement
The gulf states have been pouring money and weapons into Syria. The opposition is funded and armed to a significant extent by KSA and Qatar. That is a key reason why the opposition is quite so toxic - much of the heavy lifting is being done by people we would happily describe as "al-qaida" if they popped up anywhere else.
Turkey has been permitting opposition fighters to base on its territory, and has been providing some logistical support I think. I'm not clear what Turkey's perception of the risks created by partition of Syria and a potential link-up between kurdish syria and the kurdish autonomous region in Iraq are.
Russia supplies weaponry and credit to the Syrian government, which provides Russia with warm-water naval facilities that aren't locked up behind the Bosphorous. Russia has modern warships in port in Syria at present.
Iran has been funnelling weapons and personnel into Syria to support the government. Iran needs Syria friendly as its conduit to hesbollah, apart from the importance of avoiding isolation.
Large numbers of hesbollah fighters from Lebanon appear to have been fighting with Assad's troops.
Israel is staying pretty quiet. Iran has threatened to retaliate against Israel if Syria is hit. Israel's disputed border in the Golan has been reasonably quiet, although Syria facilitates hesbollah's confrontation on the Lebanese border. I suspect the Israeli calculation is that a KSA-sponsored shambles with a government looking something like Hamas would be worse for its security than the ba'athists are.
China and Russia, fairly crucially, are actively against anyone intervening. The Chinese are pretty absolutist about the sovereignty of states (understandable both due to their past history and also to their current government arrangements) and Russia has always defended its right to deal with secessionist territory as it sees fit.
Sorry for the brain dump. The maxim "gassing people is bad" is clearly correct. Applying it as policy to Syria is almost impossibly hard.