The little analysis I've seen of this so far has focused on how it is a blow to American power in the area, or at least an attempt by Russia to block American power. However, I find it much more likely to be a check on Chinese power in the region. Central Asia already had a multilateral security framework for dealing with the problems of terrorism and extremism (two of the three targets of the CSTO force) in the Shanghai Cooperation Organization. Moreover, the SCO is made up of many of those in the CSTO, only lacking Belarus and Armenia.
It was no secret that China did not support the war in Georgia in '08, due to its long anti-secession stand. The two have also had plenty of disagreements over oil and gas contracts in Central Asia and elsewhere, and China has already completely eclipsed Russia as likeliest competitor to the US on the global stage, with the "G2" having eclipsed both the old "US-Soviet" summits and even the G8 as the biggest deal in international meetings. While I personally think the power of China has been completely overblown, it would be natural for others to try to balance against that power. In particular, Russia would hate to have to be a weaker ally of either China or the US in a new bipolar system. The CSTO could be an attempt to forestall that with its own, non-Chinese alliance.