I have summarized the process of a decline of Polish-Lithuanian Commonwealth in this answer that somewhat complements the information provided below, so you might be interested in the former as it also touches upon the subject at hand.

The whole concept of a 'golden liberty' was able to thrive in this or that shape chiefly because it had a wide support of the entire noble class that rose to the dominant political power in Poland and Lithuania and then in Polish-Lithuanian Commonwealth thanks to the vast privileges received in 15th and early 16th century. Privileges gained by the knightly class in the mid-to-late 15th century allowed it to exert stronger pressure on king, slowly sidelining position of the latter. This was best exemplified in the proclamation of 1504 that forbade the changes in the management of the royal domain without the approval of the parliament and especially in the quasi-constitution of 1505 staining that no new statewide laws can be introduced without the approval of the nobility, essentially reducing the position of the king to that roughly equivalent to head of the executive branch of the government. Such solutions were largely supported by nobility as it has been considered by the latter as a good preventive measure against the power of the king or wealthiest nobles. And indeed, in the first half of 16th century this actually helped to eliminate some abuse of power by the high-ranking officials in the so-called 'executionist movement'. The idea of 'golden liberty' was additionally reinforced by the Union of Lublin signed in 1569, as the strong position of nobility was a factor unifying the multi-ethnic, multi-cultural and multi-confessional country into a sort of 'noble nation'.

As the strong position of the nobility as well as the peaceful coexistence of various groups was a fact since 15th century, the idea of 'golden liberty' was largely a highly conservative ones, with the nobility aiming to preserve that state rather than introduce any serious reforms. This was reinforced in 16th century with the emergence of absolutist monarchies across Europe what only entrenched the idea that the liberties should be protected from the central power of the king at all cost. Thus, the Polish-Lithuanian Commonwealth existed as a form of federation of the areas characterized with a large extent of self-governance that generally saw it more dangerous to create new laws than to not introduce any change. This was used by the wealthiest magnates to increase their position in 17th century, after the series of destructive wars that impoverished large part of nobility, weakening its position and rendering the situation in the country hard to rectify.

It should also be noted that between mid-15th and early 17th century, Poland and Lithuania shared borders with the states that were either not adequately powerful or developed to significantly endanger them despite numerous wars. Russia was only emerging as a power after a period of Mongol dominance, Holy Roman Empire was highly fragmented, Teutonic State was effectively a Polish vassal state, Sweden was largely a poor country struggling after the end of Kalmar Union and the Habsburgs only began to consolidate their power in southwestern Europe. But as the aforementioned countries began to develop, forming highly efficient governments and diplomatic services in late 16th and early 17th century, Polish-Lithuanian Commonwealth, already within the grasp of the wealthy magnates, became a kind of diplomatic battlefield of the foreign influences trying to bring this or that magnate to their side (especially during the royal elections, as Polish laws did not forbid a foreign candidates to become contenders for the Polish-Lithuanian throne), slowly eroding the strength of the country that was already diminished by the relatively archaic, largely feudal economy and low level of urbanization. All these factors turned the the conservative approach that previously helped to upkeep the 'golden liberty' into a major obstacle preventing any rectifying reforms that could have helped the situation in the country.

Could anyone vote? Technically, yes. Everyone belonging to a Polish or Lithuanian noble class, whether a native or a person who received the nobility through indigenate, had a full right of participation in any facet of political life, with no division along the lines of wealth or influence. It should be noted that Polish-Lithuanian nobility did not really introduce a concept of divergent of noble titles associated with formal political abilities, i.e. all noblemen were nominally equal when it came to politics.

Speaking of voting, I think a little explanation is in order. The voting itself, understood as a process of acceptance of the proposed changes to the statewide law or important decisions impacting the entire country was a relatively rare event, somewhat limited in scope. It was only undertaken during the Great Convocation that was a six-week long session usually called every two years. Discussions and eventual voting was carried out by the representatives of the nobility from every land (province), the senate, composed of high-ranking state officials, and the king. Of course, the representatives were also elected to their positions during local convocations, where local nobles were choosing their representatives, usually selecting a dozen or so candidates and then openly voted to choose two that were entrusted with the representation of all other nobles from a given land. Technically, any single noble had a right to voice their opinions and be elected to be a parliamentary representative or hold a local or central office.

Nobility was a relatively closed estate, although in 15th and 16th century many representatives of the urban middle class, especially richer ones, were able to enter this estate by marriage or purchase of land. This was generally criticized by the nobility as such, and thus from 1578 a person could have been ennobled only by the Parliament and by king during the wartime, although only with the approval of the representatives of the army. The latter instance became quite common in 17th century that saw multiple conflicts, causing the nobles to suggest complete closure of the estate. This have never happened, although in 1669 the instance of scartabellate, of 'probatory nobiltation' has been introduced. People receiving such distinction were considered nobles, but they, as well as their children and grandchildren were unable to hold offices and become the parliamentary representatives.

So, to sum it up, the 'golden liberty' existed to a substantially long time because of the relatively high level of self-governance by the numerous nobles, who, being characterized by a high level of political culture managed to operate the country in an efficient way for a time being. But as the foreign powers gained power and the Polish-Lithuanian nobility lost it to the wealthiest members of the noble estate, the same conservatism and belief in self-governance hampered any coordinated effort to introduce reforms that could have reinforced central government and enable it to counteract the actions of the neighbors or introduce statewide economic reforms to strengthen the country as such.