One of the major differences between the Tlaxcallans and the majority of their Nahua speaking neighbors was that their political system was based on meritocratic rule. They were essentially a primitive republic ruled by a council of 50 to 200 chiefs who were chosen for their service to the state. Although their society was divided into noble and commoner classes those from the commoner class could (and did) serve in the ruling council, generally through military success. The Tlaxcalans were fierce enemies of the Mexica and became a key ally of Cortes in his defeat of the Mexican Empire. Fargher et al argues that the Tlaxcalan system of egalitarian and collective political rule was the driving force for such a fierce resistance to Mexican domination. Restall argues that without Tlaxcalan assistance the Spanish may not have been able to defeat the Mexica (or at least it would have taken much longer). Cortes' alliance with the Tlaxcallans was the primary factor behind such a fast conquest. Because the Tlaxcallans helped Cortes they generally held a higher satus in the social order after the completion of the Spanish conquest. If you have access to JSTOR I will put the link to Fargher's article here. If not then here is the citation: Lane F. Fargher, Richard E. Blanton and Verenice Y. Heredia Espinoza, “Egalitarian Ideology and Political Power in Prehispanic Central Mexico: the Case of Tlaxcallan,” Latin American Antiquity 21, no. 3 (September, 2012).

Edit: You may also want to check out this article as it seems to be relatively well cited.