The comparison between Mr Juncker's "election" and that of the UK Prime Minister is inappropriate. To be pedantic the UK Prime Minister is not actually elected by the party he or she leads. Following a General Election the Queen invites whoever she thinks is most able to form a government of ministers to do so. But being practical when voting in a UK General Election voters know who the candidates for that invitation are. Whoever that person eventually is will be accountable, via MPs, to the House of Commons.
However, Mr Juncker is not being proposed as Prime Minister of Europe. He is being proposed as the President of the European Commission. This body consists of unelected commissioners and has the sole power to frame and propose EU legislation, to enforce compliance with EU treaties and to shape national budgets. The elected representatives whom people voted for a few weeks ago (MEPs) have no such powers.
When people went to the polls in early May I doubt that anybody outside Luxembourg had ever heard of Jean-Claude Juncker and many in the Grand Duchy probably wished they hadn't. His CV is interesting. He was one of the principle architects of the single currency (which he may like to keep quiet about). His committment to an open and democratic EU is perhaps questionable. Ahead of the French vote on the European Constitution in 2005 he said: "If it's a Yes, we will say 'on we go', and if it's a No we will say 'we continue'". In 2011 he said "monetary policy is a serious issue - we should discuss this in secret, in the Eurogroup. I am for secret, dark debates".
For me he sums up everything that is wrong with the EU. Anti-democratic, secretive, contemptuous of the electorate. But that's the trouble with democracy (EU style). Everybody gets what nobody has a say in.