• What impact does the copyleft have on choosing a software licence for one's project? 

This is a short overview of different types of copyleft in FLOSS licenses. Essentially there are four types of copyleft. No copyleft, weak copyleft, strong copyleft and very strong copyleft.

No copy left : Thirdly we have licenses that are short and simple permissive. The only conditions being preservation of copyright and license notices. Modifying and distributing can be done without releasing source code and for some cases stating changes. Such licenses are for example the MIT, Apache and BSL. Software under MIT license is for example Ruby on Rails and Node.js. 

Weak copyleft

The main difference between strong GPL and the lesser variant is, that the lesser one allows the creation to be linked with a non copylefted project. The non copylefted software can then be redistributed under own's terms if it is not a derivative work, otherwise the software's terms must allow modification and reverse engineering for debugging such modifications. Weaker copyleft licenses are LGPL, MPL and others. Software licensed under LGPL is 7-Zip and some VLC media player libraries.

Strong copyleft. GNU GPL is a strong copyleft license which is usually used as the baseline for comparison with other copyleft licenses. It mandates that the code of derivative work to be released under terms of the same license and does not permit compilation with differently licensed code. Linux kernel is example of GPL licensed code.

A controversy was caused by incompatibility between CDDL and GPL. These licenses share the same spirit of copyleft but because of GPL’s strong copyleft, OpenZFS cannot be easily combined together with Linux.

GNU AGPL is very strong copyleft license. The only difference from GNU GPL is that network use is treated as publication. This license is targeted at hosting providers that run modified versions of copyleft software as a service and keep modified code to themselves.[2]