As far as the most serious obstacle to democracy, it's the democracy process itself sometimes. Meaning, enough people can vote for something, including some horrible ideas, and it strips a group that doesn't have the organized clout of its democracy. Slavery for example, was really popular, but that democracy concept didn't work out for everybody. More subtle forms of the public voting for politically popular concepts which create a Big Brother and Nanny state of protecting people from themselves pose a great threat (i.e. Proposition so and so in California banning whatever is popular to ban at the time). I hate to use the cliche "never underestimate the power of stupid people in great numbers", but that poses a huge threat to democracy with knee jerk reactions leading to unconstitutional legislation (and legislators perfectly willing to go along with whatever gets them the votes).