China and Russia vetoed the UN draft resolution condemning the violence in Syria. My bet is that one item in particular in the resolution was the deal-breaker. Let's go to the text, section 7:
7. Fully supports in this regard the League of Arab States' 22 January 2012 decision to facilitate a Syrian-led political transition to a democratic, plural political system, in which citizens are equal regardless of their affiliations or ethnicities or beliefs, including through commencing a serious political dialogue between the Syrian government and the whole spectrum of the Syrian opposition under the League of Arab States' auspices, in accordance with the timetable set out by the League of Arab States;
And there you have it. The Arab League supports a "political transition" to a democratic system. Meaning, after a sustained protest movement with a harsh government repression, the Arab League wants Assad to step down and a democratic system to be implemented instead (The two cannot coexist; Assad and democracy are mutually exclusive systems).
Of course China and Russia would veto this, at it is directly related to their respective domestic political systems. Both China and Russia have politically repressed systems and don't want their populace to get the idea that after a sustained protest, it is possible for an outside collective to push for democratic change. Plainly, it is not in their interests to promote democracy because they don't want it themselves.
However, to call this "a resolution condemning the crackdown in Syria" (as the BBC puts it http://www.bbc.co.uk/news/world-middle-east-16892200) is misrepresenting the text. While the draft is full of condemnation for the crackdown, it contains the above quoted excerpt. Therefore, in addition to condemning the crackdown, the resolution also calls for a change in political systems (read: regime change). That's not something to take lightly. Voting for this resolution is showing support for a change to a democratically based system.
This isn't about Syria. Russia can sell guns to plenty of other clients. This is about regime stability. This is about outsiders influencing domestic issues. This is about supporting democracy. Russia and China don't support democracy nor outsider influence in (their) domestic affairs; the UN draft resolution does. You do the math.
(In the interest of fairness, let's not forget that the US [solely] vetoed a resolution condemning Israeli settlement building and its failure to abide by multiple UN resolutions of the past. Check out the text:) http://en.wikipedia.org/wiki/United_Nations_Security_Council_Resolution_446#Text_of_resolution