This is a picture of a dryer vent that could lead to bad things in the future. There is a grid on this particular vent, but there are many different styles on the market. This vent is obviously catching lots of lint from the dryer. This can cause excessive heat in the vent that *could* cause lint in the line to catch fire. It will *definitely* make drying your clothes take a lot longer because the air won't flow out like it's supposed to and will make your machine work harder trying to push the air out...thus making your dryer wear out faster. The machine having to work harder and/or longer could also be felt on your electric bill due to overworking the appliance. Not only vents with grids on them cause lint to build up. Lint builds up in most dryer vent lines.
"Okay, But How Can I Prevent This?!"
Easy! Clean your dryer vent lines at least once a year! If you aren't comfortable doing it yourself, or you just have no clue how to do it, then hire a professional to do it for you! You could save time, money, and possibly your house!