In the course of the development of institutions having their foundations in theoretical thinking, the most significant contributions are those which not only, or necessarily, give the theories substance, but those which give the theories a form consonant with the realities and serving, by this form, to further promulgate the institutionalization of the theory. The implimentation of a theoretical system does not necessarily imply that the form thus constituted will work in reality or continue to function once instituted - even if the system is of a nature such as to be consonant with the needs of the situation. Thus, the form devised to embody a theoretical system must, in addition to consonance with reality, be self-serving.
The best Solon could do, having recognised this principle and seeking to insure the maintenance of his system, was to declare a moratorium on constitutional changes and leave town. His system did not have, inherent to itself, a form such as to perpetuate the theoretical system in constituted; it needed to rely on rule appended to its form and function to assure its continuance. This is not to say that Solon's contributions were negligible, but that, though these contributions were applied, they were isolated from the process of formation qua process (although they may be considered as major contributions, especially in respect to the setting of precedents). By his method of implementation, however, Solon did make a contribution to the science of applying an ideology if only in his grappling with the problem of such an application and thus pointing up the intrinsic difficulties.
The most significant contribution made to the formation of the Athenian democracy of the late fifth century B.C. were those of Cleisthenes. These contributions were predated by others of a similar self-serving aspect, but alone of the various contributors mentioned did Cleisthenes appear to recognise the need of the form of his changes to be such as to contribute to the development of the theoretical system. Because of this recognition, Cleisthenes contributed most to the rudimentary workings of the system and, thereby, to an assurance of its endurance.
To follow the question one step further and seek a single feature contributed by Cleisthenes, is my next task. An interesting consideration is that of the law of ostracism, but, while this did act as a safe-guard to Cleisthenes' system, it is no less an appendage than Solon's moratorium; though not quite so obvious and direct.
The single feature of Cleisthenes' contribution which is most in keeping with instituting and maintaining the theoretical system is his conception of and application of isonomia. Most notable in the application of this feature is Cleisthenes' utilization of the demes, phylae and trittyes "to bring together in phylae men of very different social standing, above all, the urban and rural populations. The general description by Aristotle that Cleisthenes wanted 'to mix the people,' and thus to create a unified body of citizens, fits the traceable facts best. His laws were called nomoi as an expression of nomis imposed on and by the people ... That is also confirmed by the name which Cleisthenes gave his new order: isonomia, equal distribution and thus equality among the citizens, equality before the law as well as equal political rights, equal share in the state." (Ehrenberg, pp. 95-96)
The proof of this concept as abetting, by means of its form, the implimentation of the theoretical system of democracy (which was henceforth refined in particulars, but herein had found its essence) is cited by Ehrenberg: "Within that mixed society which Cleisthenes had created and which was most effective in the prytaneis, deriving from one phylae only, democracy began to work almost at once." (p. 99) "New forces were released when new strata of society emerged. There was a first flowering of a specific Attic art ... Cleisthenes' isonomia, equality of rights among all the citizens, was not yet rule by the people, that is democracy, but it was speedily approaching it." (p. 102)
This one feature, then, of isonomia as implemented by Cleisthenes, may be said to have been the most significant contribution to the formation of the Athenian democracy of the late fifth century B.C.; both in the fact that it embodies the essence or spirit of democracy and in that it provided a framework within which the evolution of democracy could be accomodated and which accomodated that system's self-preservation.
(All quotations from Victor Ehrenberg's From Solon to Socrates)