One big draw to using an SLR type camera is being able to buy and use many different lenses. Most people starting out with their first serious camera plan on getting several lenses, often wishing to cover every focal length from super wide to very long. It is very fun to collect new toys to use; however, there are many advantages to taking a simpler approach and to use and own only one lens. I currently am a one lens shooter mainly from necessity - I decided that I wanted one good lens and I didn't have the money to buy another. Along the way, I have ended up finding many reasons why it is more productive and enjoyable shooting with only one lens:
Using more than one lens means carrying the extra lens or lenses. This usually means resorting to some sort of back like a backpack or (gasp) a hip or fanny pack. I find it much more enjoyable to take pictures carrying as little as possible; one camera/lens held in one hand. No bag. If I am trying to take some "serious" landscape or macro pictures, I might carry a tripod and stick some filters in a pocket, but I try to be portable and not use a bag in the field. For street or urban photography, a bag can be a giveaway to what you are doing. The freedom to walk with only a camera grasped in my hand is much less obtrusive and I can quickly raise it to take a quick picture.
So much goes into getting a good picture of a subject - exposure, lighting, composition. Add to this juggling various camera settings (f-stop, iso, shutter speed, tripod or no tripod) and you can become paralyzed with decision making. If you have a bag of three lenses, then there are three more options to confuse and slow down your workflow. With only one lens, there is no decision to make - you take the lens you have and make the best images you can.
If you travel with a bag full of lenses, odds are you will start by searching for an interesting subject and then try to figure out what lens to fit that subject. I have found that starting with a subject is sometimes not the best, especially with no concept for what you want the end result to look like.
Having one lens forces you to think in terms of what subject will best match the lens focal length you are using. At first, you will be frustrated by seeing all the pictures that you can't get with your lens. After some time, you will begin to understand exactly what your lens is good at and you will start to search out scenes that are a perfect match for that focal length.
As an example, if I only have a wide-angle lens, instead of looking for detail shots or far away subjects, I would instead look for compositions involving layers of elements from near to far. I would take special care to find an interesting foreground element.
One drawback to SLR cameras with removable lenses is that dust can enter the camera body and get stuck to the sensor. If you change lenses often, eventually enough dust will get into the body and specs will start to show up on your images. I have never seen dust spots in my pictures and don't expect to anytime soon. As a one lens shooter, I never take the lens off so there is no way for dust to get in.
I know my photography has benefited from using one lens for the past year and I am thankful for the new approach I have gained in searching out compositions that fit my lens. I think that everyone could benefit from a simpler approach and I hope you are encouraged to make the most of the equipment that you have.