It's always surprising to me how Americans have (so far) seemingly accepted the incredibly undemocratic features of the House. The Senate can be said to be undemocratic as well, but at least it was deliberately designed that way (and it makes a certain amount of sense. We also have "deliberately undemocratic" features in Norway, where the larger counties get more mandates per capita).
But the House should be... well, representative. And yet we're very likely looking at at least a 5% overall advantage to the Democrats and still it's up in the air whether or not they'll even have a majority.
I'm sure the two-party system has something to do with it.