You don't always have to do the "big changes" to drastically change the mood and effect of a room. Sometimes you just need to change one aspect to bring it all together have the room complete, just the way you intended it to be.
This goes for both the interior and exterior of your home. Since it is winter let's focus on interior (unless you like to do outside work in the cold!). Many of the rooms in your home require some kind of privacy. Here are a couple things to keep in mind when swapping out doors.
Is the room private (bedroom, bathroom, etc.)
Do you want glass in your door? Depending on the types of room you can go with a clear glass or a more "private" option
Depending on your style of home you may want a wood door option vs. the standard molded door.
Regardless of what route you go, and the style you are looking for, there is an option out there to perfectly fit your home.