I like using tele lenses. Not because of the fact that you can keep your distance when shooting the subject, but mainly because of the effect that the compression of the subjects in the distance. Everything looks closer together and the viewer gets a feeling to be really involved in the complete scene. With this the background plays a more prominent part in the composition of the scene and in telling the story of the image.
An other advantage of the compression is that the photographer is able to link elements of the scene that otherwise would not have been noticed together. The example of the image with the big eye on the wall looking at the street sign with Jesus on it is an example of that, as well as the image with the stroller and the pregnant lady talking to the half chopped off guy. He should have been in the frame as well!
The focal length of the lens I used for this exercise was 105 and 75 mm. I know this is not very long, but the longest at my disposal at the time of shooting.
The practical difficulties I noticed were the fact that people would stand in my view, the longer shutter speed that is needed to get the same exposure, thus more risks of blurry images. What I did like about it is that when shooting at this distance, the subject is truly unaware of me being there and therefor not reacting on me. I also find that with the close up it's really possible to focus on people's expressions, since the background is blurrier and more compressed. The subject seems to be more separated from its environment, which brings a specific atmosphere to the image.
I chose to do this exercise at a very touristy spot, so that it wasn't obvious that I was taking images and people wouldn't get upset with me either. The image of the man was taken from such a distance that he didn't notice me.
A disadvantage of a long focal length is that with taking the image you're unaware of what's happening right in front of you, so there's the risk of loosing out on other elements that would have been interesting to add to the composition. But again, for a strong juxtaposition and a strong focus on the subject itself, the long focal length really works.