most of the top level governors are not elected. despite being democratic. and the hereditary part reminds me of feudal times.
The Supreme Council is not the "top level governors", despite their grandiose name - they're the house of review, akin to the House of Lords in the U.K. or the Canadian Senate. Executive powers reside in the United Earth Council, not the Supreme Council, which is composed entirely of members of the Assembly and chaired by the Prime Minister, who is Head of Government. They have mostly ceremonial and defunct powers.
Keep in mind that this is, essentially, a Westminster system, and not, unlike the Federation, based on the American system of government.
The inspiration for having hereditary members of the Supreme Council comes from an episode of... I believe it was The Next Generation but I'm not positive... in which it is referred to that Betazed, with a formulation stating that it is in good company with other major members of the Federation, has archaic, hereditary offices with theoretical power and significant influence in their democratic government. The hereditary seats are essentially held by monarchs or descendants thereof of certain states, either because those states were influential in forming the United Earth or throwing a bone to smaller monarchies to convince them to join up. Given the explicit existence of a
Third French Empire in semi-canon Star Trek, I figure that monarchy's gotta make a comeback of some kind in the 21st century. The elected seats are an extension of this - they're not elected by equal districts or anything of the kind; they hold elected positions over certain countries powerful at the time - the elected leaders of the United States, Soviet Union, China, India, etc.
Though at one time the UESC would have had certain reserve powers, almost all of them were related to foreign affairs and war, both of which have been ceded to the Federation since that time. It is not a "true" bicameral legislature, in the American sense - the Assembly holds all real sovereignty, authority and executive power of every kind, apart from a single power: the Supreme Council preserves the ability to force a new election under certain, strictly defined conditions. Otherwise, the Supreme Council is a glorified gentlemen's club that studies and makes suggestions on changing proposed laws. As I showed in a previous infobox, former Prime Ministers are traditionally appointed to the UESC, even when their opponents are in power. That wouldn't happen if the UESC had any real power.