NATO is only being blocked by them wanting an actual mandate from the UN Security Council this time. This is what Russia and China are blocking, them being permanent members in the UNSC, with the ability to veto any proposal on the table.
NATO forces have, in the past, engaged in military operations without UNSC backing. The 1999 bombings of Yugoslavia are a prime example of that, but the legality of said event is
unclear at best.
However the Syrian situation is strategically and politically very different.
In Kosovo conflict, strategic air strikes were used to pressure the Yugoslavian leadership (Milosevic) to back off from their operations in Kosovo. In Syria, the conflict is more widespread than that, rather than Syrian forces being deployed in a particular region, they are engaged with rebel/insurgent activity country-wide.
Kosovo war was an ethnically motivated act of war against the nation's own citizens. The Syrian conflict is a case of several rebel factions (with varying motives and goals) fighting the official regime and, to varying extent, each other.
Serbian bombings had, at the very least, tacit approval of Russia at the time, rather than complete blanket disallowance. Russian units were the first UN peacekeepers in the country after Milosevic capitulated (after Finnish-Russian mediation team had convinced him to do so). The peacekeeping operations were under UN mandate, but using NATO units (KFOR) and, grudgingly, Russian peacekeeper units worked under authority of NATO troops.
Similar to both situations is the extreme reluctance to deploy significant ground troops. However in Syrian case, it is unlikely that air power alone could accomplish anything meanignful; ground presence is a must-have to get anything done. At the current political situation no one wants to send their troops there, like Luis has pointed out.
Even if by some miracle, political solution was achieved to send NATO or Russian troops to Syria to pacify the situation on UN mandate, or as peace-keepers afterwards, it would very likely become another insurgent-infested nightmare for everyone involved.
From a humanitarian point of view, I definitely agree with MP-Ryan. The conflict has only been escalating since it started, and it has long since reached a stage where outside intervention became a legitimate choice. By stonewalling ANY intervention, Russia and China share a certain degree of culpability in allowing the conflict to escalate.
However I wouldn't be so quick to unilaterally condemn the Russians on this. The opposition to Assad is fractured and, as events in Syria have shown, questionable in their motives. Parts of opposition-controlled Syria have been put under Sharia law and I doubt anyone in the West wants that to happen for the entire country. Islamic organizations (both political and terrorist kind) are sending weapons and troops to Syria to further their own gains.
It's pretty hard to justify a military intervention - particularly ground forces - when there is no real, viable political presence in the target country that you can genuinely support. When there's no plan for the future coming from within, the only course of action would be a full invasion and establishment of a new order in the country, and THAT would certainly go down well.
In a way, waiting for UNSC mandate is one indication that shows how reluctant NATO countries are to act. They've acted without direct UN approval before. In fact there's probably more of a reason for them to act now than during the Kosovo conflict, considering one of their member countries (Turkey) has come under attack from within Syrian borders several times now.
They just don't really want to get involved, which is understandable from a Machiavellian point of view, but condemnable from a humanitarian point of view.
So, if one were to believe that a military intervention is the only way to solve the conflict AND that it should have been done already, then Russia is not the only one to blame. Even without UNSC mandate, actions could have already been taken; as the old saying goes, it's easier to ask for forgiveness than permission. So, if a military intervention should have been done, then NATO countries should have done it already. Of course, there would be an interesting spectrum of possible responses from the Russian (and Chinese) governments in that case.
Also, with all the semi-recent military actions whose legality has been dubious at best - specifically, US invasion of Afghanistan and Iraq, and indeed the Serbian bombings in the previous decade - I'm not surprised that NATO countries want this one played "by the rules" - especially as it also happens to give them a convenient reason to stay as far away detached from it as possible.
So it seems that, at least for the time being, it makes no sense to me to start blaming any individual country for a military intervention that never took place. No one probably wants to do it, whether it would be right or wrong thing to do. There's no strong faction in Syria that would be sympathetic to the Western governments; more and more it's starting to seem like it's a power struggle between different islamic factions in addition to rebels trying to topple Assad's regime. And while it's pretty sure Russians don't exactly
like dealing with Assad either, any option would be worse for them (islamic or secular).
That still leaves a few different options for helping the civilian populace of Syria, those who are not involved in the fighting but would catch the brunt of it (either internal fighting or external intervention). Humanitarian support by sending food, emergency supplies, medication and medical teams is already being done, I think.
Another option would be to simply offer asylum for anyone not interested in fighting, and evacuate the country of anyone willing. But who wants to receive all them potential terrists, right?