There are two main problems with the US's government as it stands (this is a much more simplified version than reality, of course.)
1) The Founding Fathers were so afraid of tyranny and "a new king" - or at least that was their excuse - that they rigged the United States government to be so inefficient that one person could never take power. We are purposely inefficient because we're "thorough" when it comes to avoiding centralized power.
2) The two-party system is broken.
Thing is, it worked well for nearly 300 years, its just now some of the issues they didn't get rid of (gerrymandering and lobbyist are two that come to mind) have really caused huge issues. We often only get something done while its either tied to some other bill, which is stupid, or because some companies/individual throws money at people to get bills that favor them.
Two more problems also come to mind. The fact that the filibuster has now come down to stalling everything under the sun, it needs limits. The other is that there isn't a coalition government like there is with nearly all democracy's.