Answer:
The biggest one I can think of is that America operates under a two-party system: one where only two parties have the reasonable prospect of winning. For the United States, these are the Republican and Democratic parties. Other parties, like the Green party, are permitted to exist, but they never have any major representation in federal government. Because only one of the two parties have the highest chance of winning, people resort to strategic voting: a kind of voting where people pick a candidate who kind of fits their needs while still having a chance to win rather than picking a candidate off of preference.