The ability to cut through space and make distant subjects seem much closer is the reason most people want long-focal-length lenses and it's also why camera manufacturers work so hard at making good-quality and affordable zoom lenses. Long telephoto and zoom lenses are great for sports, wildlife or candid people photography or any situation where you want close shots of far-off subjects close. In a lot of situations (like sports) using a long lens is the only option you have.
One of the problems of using longer focal lengths though is that they compress the apparent distance between different areas of your compositions. While this can be a creative advantage at times (compressing a silhouette of a sailboat into a setting sun, for example), it can also create situations where it's hard to separate the main subject from the background--as is the case in the top photo. I shot this photo of a tugboat in New York harbor (from the New Jersey side of the Hudson River) using a Nikkor 70-300mm lens at 165mm (which is roughly 247mm in 35mm terms). While I like the idea of the Manhattan skyline behind the tug, from the vantage point that I had there is no separation between the boat and the buildings. Had I been able to find a better vantage point (perhaps a more head-on angle on the tug), the focal length might have been fine. But as it is, the background seems to clutter the shot too much.
As the boat came closer to me though, I zoomed out a bit to a focal length of 220mm (about 330 in 35mm terms) and waited until it cleared the clutter of the buildings and had just an open river (and in the distance the George Washington bridge) behind it. If I had had the time, I would have backed off on the zoom a bit and shot the entire boat, but because it was moving so fast and I wanted a closer shot, I had to shoot very quickly. By the time I got this shot the boat was almost right in front of me. And as I said, I really wanted a shot of the tug in front of the city buildings, but when you're shooting moving subjects you don't always get what you want.
Whenever you're using long telephoto lenses (anything that is longer than, say, 135mm in 35mm terms) be aware of what is behind your subject and make sure the compression isn't combining the near and far in an unattractive way. If it is, see if you can find a better angle so you can see some space between near and far and if not, you might have to alter your composition a bit or back off on the zoom. In this case I was lucky because the subject was moving and I was still able to use the longer focal length but choose an alternative background.