(Warning: some complaints about the neglect of ancient concepts by classical scholars).
I just read a nice piece by W. Jeffrey Tatum on "Roman Democracy?" in the Blackwell Companion to Greek and Roman Political Thought that makes a good case for taking the "democratic" features of the late republican Roman political system seriously, contrary to the common opinion among scholars since the late 19th century that republican Rome was essentially a senatorial oligarchy.
Tatum sensibly notes that Rome was not a full democracy in the strong sense of the term (pace Fergus Millar), yet he also seems needlessly dismissive of Polybius' and Cicero's judgment that the Roman constitution was "mixed" (pp. 215, 223). Rather, he seems to want to classify Rome as some kind of imperfect democracy, and in the process of casting about for a suitable concept ends up suggesting that Rome was a "delegative democracy" (p. 226). This is a term invented by my old teacher Guillermo O'Donnell to describe political systems like that of Argentina under Menem or Peru under Fujimori, but it is wholly unsuitable for describing the late Roman republic: a "delegative democracy" is a democracy without mechanisms of horizontal accountability, and the Roman republic certainly did not lack that!
Why not simply say that the best way of describing the Roman political system is through something like the Polybian concept of the "mixed" constitution, or alternatively through the concept of a "hybrid" regime, to use contemporary terminology (though the concept of a hybrid regime today is much less well developed and lacks the normative associations of the idea of the mixed constitution)? The evidence Tatum cites shows quite well that the Roman regime fits the basic ancient criteria for classification as a mixed constitution. It involved a number of distinct centers of power with with complementary but also competing interests (the tribunes, whose power derived from their connection with the popular assemblies and their ability to veto senatorial proposals; the senate, which was the executive committee of the Roman upper classes; and the consuls, whose power derived from their control of military forces in the field) which were not always equally balanced against each other, to be sure, but then again the idea of the mixed constitution did not require that its constituent parts be fully balanced. And it is true that the idea of the mixed constitution had a large normative baggage and implied a certain theory about political stability (which I am currently exploring in this project), but that is no reason to abandon it entirely.
Aside from concerns about the overwhelming dominance of the senate (which are basically refuted by Tatum), scholars tend to argue that Rome could not be a "mixed" constitution in the Polybian sense because the consuls (the "monarchical" element in the constitution, in the Polybian scheme) were senators and returned to that body, representing senatorial interests all the time. But it seems reasonably clear that in Rome the consuls' power derived not just from the senate, but from their control of armed force and their ability to extract resources from the provinces (as proconsuls after their terms); so Polybius' characterization of the consuls as the "monarchical" center of power (cf. πολιτευμάτων, Histories 6.10.6, which it seems to me could be translated as such) does not seem far-fetched.
So why the dismissive attitude towards the ancient conceptual apparatus? Rather than throw it out, why not develop it further and see whether it can be adapted to modern conditions? (Another example in the same book: the piece by Forsdyke on the idea of tyranny in classical thought. She seems to suggest that the notion of tyranny in the 5th and 4th centuries was simply a tool in the ideological struggle between the upper and lower classes in the polis, rather than an attempt to actually grasp a real phenomenon, however imperfectly, though the last lines of the piece soften the impression a bit).