The first obvious point to me is the lack of depth that you can end up with for example above, i isolated this patch of woodland because of the way the colours of the trees drew my eye but because there is no point in the photo to give you and idea of perspective it seems very flat. On the upside though it focuses on the colours in the landscape and is still a pretty good shot although under different lighting conditions the idea of depth could have been created by the shadows cast by the trees.
The two shots above are very similar in the fact that you are looking for a small part of a distant landscape that you can focus on to make a good shot, this is maybe a little bit of a draw back because by focusing so much on a small part of the larger landscape you can miss things for example with the shot of the trees coming down to meet the water just out of shot on the right hand side was a boat with someone fishing out of it, this would have added to the shot but if I'd have included it it wouldn't really have been a telephoto landscape.
I did find this view where even though it taken with a zoom lens it has something that my other shots are slightly lacking DEPTH. This is created by the pools of water because of the way they are spread across the field in a similar fashion to the fence line in one of the shots in my 3rd assignment (diminishing perspective) so it is possible to get perspective in telephoto shots its just not as easy as with a normal landscape.I think this kind of landscape shot does have its place which to me is when you want to really focus on a very small part of the overall scene because it allows you to get very close and intimate to the subject.
