All of the above are distractions that you find when photographing animals at the zoo.
Fences clearly will immediately block your field of view and it can be challenging to photograph the animals when it is a stationary object that can not be avoided in any scenario.
While enclosure glass may be transparent, there is a certain amount of glare and scratches that can be visible and make it harder to photograph the animals or even tarnish the end result.
Other people passing by can make it difficult to photograph things at the exact right time as they get in the way of your photograph and this can not always be avoided.
