Portability - This is a personal choice but it is worth mentioning that telephoto lenses can become very heavy and large, although the trend in modern times is to make these lenses as mobile as possible. Features that make a lens heavier and larger include increased focal length and faster apertures, which you may need to best capture the photographs you are after. Just make sure you check out a lens's weight or dimensions before you make your purchase, or better still, head to your local Ted’s store to try it mounted on a camera body.
Crop factor - If you are using a camera that does not feature a full-frame sensor, such as an APS-C format camera or a Micro Four Thirds camera, the focal length of your attached lens will be magnified. This is known as crop factor. While crop factor is not necessarily a bad thing, in fact, the extra length can obviously come in handy when using telephoto lenses, it means we use the wide reach of the lens. For example, if you purchase a 70-200mm zoom lens for use on an APS-C format camera, the lens will actually provide a range of 105-300mm; a handy focal length, but lacking some of the medium telephoto range that makes this lens such a popular choice.